I use a special mouse and keyboard (requiring less force and movement) and support for my arms.

You can easily adjust the size of the fonts on your screen, the contrast and the brightness. I have found that relatively dim brightness, high contrast and large fonts work best for me.

For me all this made the difference from being able to use the computer for less than an hour to being able to work for a few hours.

I also take frequent rest periods during the day.

I have learned that stopping for 15 minutes at the first symptoms (before they become truly bothersome) is much better and more efficient than waiting for a full blown crash.

Also alternating between different muscle groups to accomplish the same task.

Many times I rest without any body noticing that I do. In fact the best way to do that is to let someone talk to you. Listening requires very little muscle strength.