I have a G15 keyboard myself (first gen, oh yeah!), though I use either an MX518 or predecessor to DanM's trackball (ball is used by the fingers, instead of the thumb).
While the G15 may not be a nearly-zero-force Apple keyboard nor an ergonomic one, it's still not a bad keyboard, and it should be sensitive enough that you don't have to slam your fingers down on it to get it to type (if you do, I recommend disassembling and cleaning it). I find it's large enough and I use the numpad and home/end/etc keys enough that I get some good movement even while I'm just typing.
DanM said just about everything I'd recommend regarding posture. Though, I do prefer the top of my monitor to be a little higher than eye level (basically, when I'm sitting properly and looking straight ahead, my center of vision is about an inch or two into the viewing area of the screen). Also, if you don't have wrist rests for your mouse or keyboard, make sure to make a conscious effort to act as if you did. On both your mouse and keyboard, you should keep your wrists straight (but not stiff) and your hands high enough that your finger pads rest on the home keys with enough of a curl to them that you straighten to get to the next row up, and move slightly to get to the numbers. The overall position is probably like how you were taught to play piano.
As for exercises, anything that makes you move your arms and wrists differently than what you do at the keyboard will help you. If you spend all day every day doing certain motions, the muscles that aren't used start to atrophy, and your muscle strength and ligaments will distort and be uneven. This is rather subtle in the arms and wrists, but can still happen.
A more extreme example is a woman who wears high heals all day, every day. After a long time, the muscles and ligaments in her calves will shorten and it will actually hurt more to walk in flats or barefoot than in heals.
If baseball doesn't work your wrists enough, check out Yoga (Yoga's awesome, anyway, and probably wouldn't hurt to work into your daily routine, regardless), or even the wrist and arm movements taught in bellydancing (the wrist rotations are great for alleviating stress on the wrists, and the "snake arms" are a hell of a shoulder and arm workout).