I think touch typing mostly means finding the right keys by touch/memory. But it does imply using good method, which could imply using correct force. Don't make a conscious effort to avoid bottoming out, just remember you don't need to do so.
A good position should be as relaxed as possible. That normally means upper arms hanging vertically by your side, forearms horizontal, wrists straight. But if you keep the chair tilted adjust accordingly. You probably need the keyboard legs extended, but if you were sitting higher with forearms completely horizontal it would be better to keep the legs retracted.
Don't centre the keyboard in front of you - that would shift the main typing keys off to the left. Instead, have the B key centred in front of you. (Assuming a UK/US layout.)
It's best to float your hands over the keyboard, but if you must rest the weight, use a wrist rest high enough that it keeps the wrists straight in the process. If you really put weight on your wrists it might be best to get a softer (gel?) rest, but make sure it doesn't sink too low. Or use a harder wrist rest and sweat bands around the wrists for long sessions? (I've been meaning to try that.) As mentioned before, resting elbows on the armrests can help. Switching weight between armrests and wrist rest would be good.
