In my life as an coorporate interior designer I've become quite well versed on ergomincs...
Try the following:
Get a GOOD keyboard tray that has an attached mouse platform... keyboard trays are best positioned when closest to your lap. this puts the arms, shoulders, wrists and elbows in what is called the neutral postion. It allows all the tendons and viens to be unpinched and relaxed, therefor no pressure or pain. NEVER tilt your keyboard toward you, it should be on what is called a negative tilt - leaning towards the desk or monitor. When it is tilted toward you, look at how your wrist bends. Then do the same thing when it tilts away from you. Notice how your wrist extends an bends hard when tilted toward you. Away from you, the wrist is in a more natural postion. You want your wrist to have an arch under it - this allows the tendons to be unrestricted and you could type much longer. You also want the keyboard tray lower necause it keeps your arms closer to the center of the body... if they keyboard is up high, you have a tendency to swing the elbows out - causing pain after long-term use. Please PM me if this doesn't make sense and I can go into better detail.

Second, you must have a chair with good back/lumbar supprt. You must ALWAYS have you feet flat on the floor and your back resting against the support. I know the tendency to is lean forward and wrap your anckles around the base... this is a no-no. You'll notice a difference within one day if you keep sitting properly.

Now, for the proper monitor position... sit with your feet on the flor, keyboard in position and put your arm out straight in front of you (don;t strech it, do it very relaxed.) Your middle finger should just touch the middle of your montior.

Also it is important to take at least a one minute break every hour - bathroom, watercooler, whatever. Just get up, walk around and relax for a minute.

Also about keyboard trays, those ones with the extra gushy gel pads aren't as good for you are people claim. Peole tend to feel comfortable when using them but continue their bad postional habits, therefor causing more damage in the long term.

PS: it sounds plain wrong to move the keyboard tray to where I'm recommending, but believe me, it works. I've had people fight, kick and scream that they won't do it. Then a few weeks later, I' get glowing reports of how their pain is gone and how much easier it is to type now! It just takes a little time to get usd to it! Oh, also, if you're short, then get a foot rest that rocks. Everything I'm talking about is in Staples or Office Max catalogs.