I've been dealing with RSI injuries off and on for 6 years. My job means constant use of a keyboard, and at one point I was off work for 5 months trying to heal.
I've been through bio-feedback to determine my best positioning, and have had ergonomic guys at work do my keyboard, monitor and mouse to fit me.
A lot of good advice so far, particularly about keyboard height in relation to arm positioning.
Also note that your monitor should be read at such a position that you are looking straight at your monitor, or slightly higher, rather than down. Your head is heavy and puts additional strain on your neck and shoulders when tipped forward.
I'm using a special mouse that uses arm motion as opposed to your forearm muscles, and found that it literally saved my job, its by 3M. There are other special mouses out there, as well as key boards etc.
You literally also need to be in good physical health to work at a keyboard all day, some people are more susceptible to RSI and carpal tunnel than others. Keeping your muscles in your arms, shoulders and upper back strong and pliable will help you to work and play on keyboards for much longer. Don't forget to take micro breaks regularly to relax the muscles in your arms. Also, slow down a bit in your keystrokes, that can help as well.
Here's the mouse that I use...
http://solutions.3m.com/wps/portal/3M/en_US/ergonomics/home/products/ergonomicmouse/
