I actually went to an ergonomist because we had a compulsory meeting with one at school. After we'd checked my guitar playing and drums (turned out I had bad habits in playing both
) she told me some pointers about working with a computer in long sessions, one of which was that those gel filled mousepads are actually a bad way to try to solve wrist problems. Apparently if you feel you need one because of pains or whatnot, you should preferably just change the way you're sitting, whereas if you get one of those pads, you're just making your problem less noticeable, but not go away.
The correct sitting position is such that your hand goes straight on the table, so that your elbow outside the table is on the same level as the table top.
Since I started doing that, I've had no wrist pains even though my gel mousepad broke (the goo started to ooze through it which was really nasty); I've just been using a normal one.