Some wrist rests show great ingenuity
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(http://img41.imageshack.us/img41/5329/wristrest1.jpg)
The wrist rests are ingenious, but your posture isn't. Your problem isn't that you need a wrist rest. You need to improve your sitting posture and a keyboard tray will likely help.
Both your mouse and your keyboard appear to be a good two feet away from your body. This means you have to reach in order to use your input devices, which is not a good thing. Your shoulder blades are protracted forward and forcing you to slouch forward in order to reach your devices.
If you were trying to open a jar with a lid that was totally stuck, would you hold it two feet away from you while you were trying to open it? Not likely. You'd have it basically against your body where you have the most leverage and control... typing isn't quite so strenuous, but the same basic principles apply.
Elbows at 90 degrees. Elbows at your sides. Elbows almost touching your rib cage.
Always
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(http://farm6.static.flickr.com/5190/5687420702_fcdd9eb503.jpg)
I actually hadn't used a wrist rest for many years because I was using low profile Apple keyboards. Switching to higher profile Filcos was stressing my wrist and actually felt relatively uncomfortable. I hate crappy gel wrist rests but these things are amazing. Now the board truly feels as comfortable as it should be and gives me a gentle slope so my wrists aren't bent at all.
I'm thinking about getting one of these but they're quite expensive.
Do you think it was worth the money?
I haven't ever had problems after using computers but now I've been feeling tired / stiff in my arm for two days despite not having used the computer since then =(