• Before you paint: do some preliminary research on the colors you like, on the paints you find interesting, and on how those paints look / how they are to work with. 
  This is done by googling paint brand + name as well as like "Rust-oleum Primer for plastics on ABS" etc.  It is important to do preliminary research, because it can help you learn what your options are based on your budget and location, as well as any intricacies about painting with the paints you choose.  Some get very poor reviews on auto and hobby painting forums--those are the ones that come up most frequently in Google searches.  Also have a look at store reviews--some paints have Amazon and Home Depot reviews.  Moreover, you will find some interesting tips and techniques while doing your preliminary research.
  
  I cannot help you finding the best paint available in your country, but Google can!
  

• Be careful about your paint selection: the paint you like may not be suitable for your project
  And by that, I mean some paints are not colorfast if exposed to daylight frequently (some dayglow and fluorescent paints), some are not very durable; some are thin, runny, and don't cover well; some depend on the color of paint under them to influence the outcome (chameleon paints), etc.  Again, research the paints you like by looking for reviews and forum posts on automotive and hobby forums (Google Images and Google Web Search are very helpful for that).
  

• When you get your painting products in your possession, make sure to read the labels on the cans carefully and follow the instructions.  If it says to wait at least 5 minutes between consecutive coats--please wait at least 5 minutes.  Paints must cure for proper hardness and adhesion, and improper application can stall or prevent proper curing, causing your paint to lift, crack, peel off, etc.  Painting in high humidity also has adverse effects on curing as well as the actual process of spraying an aerosol cloud of paint particles onto a surface.  You may end up with a cloudy uneven finish.
  

• Look up some videos on proper painting technique. 
  That is, if you're using a rattle can or a spray paint gun, you need to make sure to paint in a gentle sweeping motion, say left-to-right and back in zig zags, and don't spray over a single spot for longer than a moment or you will spray too much!  It is better to spray too little and go over a bare spot a few times again later than to spray too much, trying to get an opaque coverage in one coat.  Spraying too much results in sagging and inhibits proper curing.
  

• Prepare your work space!
  Painting with spray cans is messy business.  Clouds of tiny aerosol particles of paint will form around the area you're spraying, and those clouds are 1. not exactly healthy to breathe, 2. can cover you and your clothes with paint over time, and 3. will make the surroundings dirty.  So use plenty of cardboard to cover your work area and preferably spray outside or in a well-ventilated garage, away from any valuables (or with valuables concealed with tarp and cardboard).  I am not responsible for your spouses / significant others becoming irritated with your newly discovered painting hobby
  

• Practice on some scrap / things you don't care much about before moving on to your keyboard case or a computer case (a.k.a. valuable item you wish to paint)
  Practicing is Always good.  The more you paint, the better feel you will get for the paint you work with: how it comes out of the nozzle, how long it takes to dry, what the finish is like, how well the paint adheres, and how hard the final finish is.
  
  Gloss paint generally takes significantly longer to dry and requires more time between consecutive coats vs. matte paint of the same brand / kind.  Some paint will take you by surprise and will continue being smudgy for hours upon hours after painting (such as Rust-Oleum Universal line)--it will take up to 2 days for you to be able to handle your painted piece without putting a fingerprint dent onto the paint.  Some paint can be relatively safely handled 2 hours after painting.  Some paint needs some sort of an adhesion promoter / primer under it to stick to plastic, and some already comes pre-mixed with a primer (Rust-Oleum Painter's Touch 2X, for example).  I've also found that some metal flake paints need a fresh base layer of paint under them to adhere to plastics (for example DupliColor Metal Specks: I spray a thin layer of something like Rust-Oleum Painter's Touch 2x under them for adhesion). 
  
  In summary, make samples first, get a feel for your paint--move on to your main project once you feel confident.
  

• When you start painting, each time you haven't used a particular can of paint for a while (or it's the first time you use it), after shaking the can well, spray elsewhere other than on your project to get the flow of the paint going properly.  Then spray onto some scrap cardboard or plastic to see if the paint comes out evenly.  Sometimes a nozzle is bad, and you'll get paint running down the can onto your fingers.  Or worse, you will get larger droplets of paint onto your project, that will look like small splatters among the finely sprayed paint.  Cleaning the nozzle with a paper towel or an alcohol-soaked cotton ball periodically helps, if it's one of those poorly-spraying ones.  Sometimes it's best to take off the nozzle entirely and give it a good cleaning with some alcohol.  If any paint has dried to cover the hole of the nozzle, you will get an uneven spray.  It can even just be a poorly-designed nozzle to start with, and you should use something else.
  
  Furthermore, some paint starts out with small caked bits that end up shooting out of the nozzle and sticking to your painted surface.  It looks terrible--like small particles of dirt or lint trapped under paint.  Usually shaking well and regularly is great to help with this, but some paint is just bad, and you won't be able to get rid of those little balled up congealed pieces.  Better to scrap that can and get a new one.  This is another reason for painting onto something unimportant first.  Imagine your surface is all nicely prepared and already painted with a few layers of paint.  Then you come back, spray some more... and get a bunch of tiny pieces of gunk onto the surface!  You'll have to stop, wait many hours, do some sanding to even out the surface again, clean, wait for the curing process, then spray again.  (This has happened to me with a can of Rust-Oleum Universal paint in Pearl Mist, for example.  Other cans in that range have been just fine, so I assume it was that individual can that had the problem).
  

• Layering paint:  make sure you read / research carefully which paints you are getting.  Their composition varies, and they usually fall into these common categories for rattle cans: acrylic lacquers and oil enamels for general purpose rattle cans. 
  There are also other varieties of paint which are intended for automotive painting and are generally more expensive: acrylic enamels (enamels are harder than lacquers), acrylic urethanes (1K (1-stage) and 2K (2-stage, multiple components)) and they usually require more preparation, such as an adhesion promoter, an epoxy primer for urethane paints, an appropriate urethane top clear coat, etc.  Urethane paints are also dramatically more toxic, and you must have good ventillation while spraying as well as wear proper full-face masks and protective clothing.
  
  Anyway, for most people here, acrylic lacquers and oil enamels will be the paints of choice, as they come in easily accessible rattle cans and don't cost a mini fortune.  Oil enamels are what I personally have the most experience with due to accessibility on-line and in automotive stores + Home Depot.  I've seen recommendations for VHT and Duplicolor Fabric and Vinyl Dye sprays, but have not tried them myself to comment.
  
  Note that acrylic lacquers and oil enamels are not really rubbing and finger-oil-proof.  So say if you paint a mouse and clear coat it and all that, you will still see the paint will begin to wear off in a month or so over the click buttons.  For touch-proof frequently handled surfaces, you need to go with more professional paints, such as 2K urethane paints.
  
  Now: oil enamel can be layered over oil enamel and over acrylic lacquer.  Acrylic lacquer SHOULD NOT be layered over oil enamel.  This applies to colors and clear coats and is important for proper curing as well as avoiding dissolving and lifting your base layers when spraying new paint over them.
  

• Surface Preparation: it is important that you inspect your piece that you will be painting and note if there is any existing coating on it, such as paint, rubber coating, anything else of note.
  Generally you should remove any coating with coarse sandpaper (good quality sandpaper is paramount, and make sure to send wet--with water--surfaces only) and clean down to bare ABS plastic.  If you are painting powder-coated steel (such as when modding a computer case), it's okay to spray directly over it--just scuff the coated surface a bit with some 1000-grit sandpaper and clean well with some rubbing alcohol.  See further in this thread about sanding QFR cases.  Once you have sanded your piece to bare plastic or your piece is all ABS to start with (in which case yous should still scuff it with steel wool or sand paper to promote adhesion--don't paint mirror-smooth surfaces, always scuff them a bit), clean it really well!  If your piece happens to have a lot of grease on it from something that you can't remove well, use a professional grease and tar remover (can be gotten for under $10 in auto supply shops).  Otherwise, wash with detergent and water, rinse really well, allow to dry COMPLETELY, then go over the surface with a fiber cloth soaked in alcohol.  Get a cloth that removes lint and dust--any dust particles left on the surface tend to come through the paint for a few thin layers.  (Also note, don't spray in dusty basements or similar--dust will be the bane of your existence when spray painting!)
  

• Primers/Adhesion Promoters: If your paint does not come premixed with a primer, it's a safe assumption that you should get a primer or some sort of an adhesion promoter just in case. 
  A good adhesion promoter that I've found so far has been DupliColor Adhesion Promoter, which is sold at all of my local auto supply stores (like AutoZone, Advance Auto Parts, etc.).  It sprays completely clear, and you only need a VERY thin veil of a layer, then proceed with your paint of choice after a minute or so.  It can be applied over bare metal as well and works quite well for that. If your surface is very uneven, make sure to sand it well to even it out first.  Otherwise, you may need something like a filler primer or even some Bondo body filler (STINKS horribly when freshly mixed but dries rock solid and works really well filling any holes.  Can be sanded to be level with your surface). 
  
  For basic paint jobs on like QFR covers, if you use Rust-Oleum Painter's Touch 2X, which contains a primer, you don't need any additional primer / adhesion promoter.  Just prepare / clean the surface well, scuff it, and proceed with the paint.
  

• When painting, for your first layer do a very fine veil of color, such that you will still see most of the base plastic / surface through it.  Say if you're panting with white, it will just look like a messy dusty surface over black plastic.  Then for your next layer after a couple minutes, you can do a SLIGHTLY more opaque layer.  But as mentioned before, NEVER do thick wet layers.  Only do heavy layers if you really know what you are doing and what to expect.  For any beginners, I would only recommend doing many light layers.
  

• Clear Coats: for most paints, I would strongly recommend a clear coat. 
  Clears are generally harder than colors, and they help protect your work against scuffs, minor scratches, and dullness.  For example, you will have a much harder time scratching a clear-coated paint job with your fingernail than if you just left it with color paint and no clear coat.  There are some specialty finishes and paints where you can forego a clear coat for something like a computer case or a keyboard case, since you won't be subjecting them to heavy use or inclement weather.  But still, generally better to clear coat than not.
  
  NOTE: once you do coat your work with a clear, it will be significantly more difficult to touch up.  So do all the corrections before you spray your clear.
  
  Clears come in different types: gloss, satin, matte, as well as mixed with some pearl or iridescent particles for a desired effect.  Gloss clear is the most transparent clear you will get.  It is, understandably, glossy and generally requires some polishing work afterwards.  Matte clears generally contain a sort of a milky substance that's part of what makes them matte, and you will unfortunately see your matte clears if you spray them over dark gloss base color paints.  So say if you're spraying gloss brown, going over it with a matte clear is not a good idea.  You want to look for a matte brown and then follow with a very fine veil of matte clear.  Otherwise, gloss clear over gloss brown will look best.  You can spray matte clear over LIGHT-toned gloss colors (like over light beige) to end up with a semi-matte textured effect, sort of like old school beige computer plastics.
  
  You can spray clear right after you paint your colored paint--if you're happy with how that came out.  If you wait more than an hour in between, you should wait the recommended 24-48 hours (or even longer) before adding clear coats to ensure proper curing (again, read re-coating information on your spray cans).  Acrylic paints can frequently be re-coated without any waiting window.
  
  Spraying gloss clear paint too much too fast can result in yellowing.  If you see any yellowing in some crevice where clear paint has accumulated--don't panic!  Generally (not always) the yellow tint goes away after the paint cures in a day or two.
  

• For any post-clear wet-sanding and polishing tips, please see links below.  Don't be over-eager, and let your paint dry really well before you do anything to it like polishing.  Better to wait for up to a week for some paints before using any polishing compounds.

• ***Information about sanding QFR top covers from the CM Store***
• * Information about sanding and polishing oil enamel (and acrylic lacquers) *
  (And additionally, please see someone else's excellent guide on painting with oil enamels and creating a glossy finish.
  Also please see this great summary / guidelines by DupliColor for their products--but also generally applicable to rattle cans:  http://www.duplicolor.com/pdfs/tips.pdf)
• Perils of using Plasti-Dip on plastic keyboard cases
• Fitting a CM QFR cover onto a Filco TKL
• Random painting tips
• Filco Camo -> Filco Copper
• Camo logo removal
• Standard black Filco MJT-2 TKL logo removal
• Filco "Sea Blue" Cover <-- scrapped, because the end result wasn't quite what I wanted (though very close).  Will probably re-create this to some extent on a future keyboard.
• Filco "Dark Steel" Cover (also samples of orange)
• Filco "Sea Blue" Cover v2 - final
• Filco "Starry Night" Cover
• DupliColor Adhesion Promoter and painting on bare steel
• Tamiya and Testors hobby paints
• "I'm afraid that I just blue myself" (a.k.a. Preval spray gun experience)
• Easy-Off Oven Cleaner to help with removing paint
• Full-sized Filco logo removal + laying drawer liner on the bottom of a case to dampen any vibrations / reduce resonance
• PlastiKote Metal Flake acrylic lacquer
• CM QuickFire TK : removing the indicator LED cover insert
• "Structural Paint" to restore matte appearance

Out of interest where did you get those metal arrow keys? Haven't been a fan until now but they are sexy!

• Rust-Oleum primer for plastics as a base (mixed feeling on this... but I used it just in case, as Stops Rust can dry very hard, and I can't have anything chip off later)
• Rust-Oleum Stops Rust "Gold Rush" as a base golden color (kind of a boring color on its own)
• Rust-Oleum Stops Rust in metallic blue on top (VERY nice color on its own, goes on thickly and smoothly)
• Dupli-Color Metal Specks in "Shimmering Green" as a fine mist on top (few fine misted layers)--not a big fan of this one, as it tends to splatter; have to be super careful (not a very good nozzle)