There are a few things you can do though most important centres around getting the right ergonmic design for your individual needs.
Firstly, make sure your desk/chair is at the right height. This is probably the leading cause for wrist problems. Working on a PowerBook (or any laptop for that matter) alone does make it difficult. So a few products that I can suggest to help this are…
1)
Griffic iCurve. This will plae your PowerBook's screen at the right, or at least a far better, height when you are sitting at a desk.
2) A full sized keyboard. Obviously the Apple wired or
wireless keyboards are designed for our keysets but any comfortable keyboard, even the split ergonimic style, for your needs is better.
3) A wired or wireless mouse.
PowerBooks are a compromise between portability, style and comfort so the best ergonomic placements aren't available for many people. If you do use your computer a lot at a desk I would suggest adding some products that can make it as comfortable as possible.
I used the above products to help with shoulder problems and it made a world of difference.