Like your chair, your work surface should fit you.

Once you have adjusted your chair, you can determine the appropriate height for your work surface.  The top of your work surface should be at your elbow height. Elbow height is measured while your upper arms are hanging relaxed by your sides and your lower arms are bent at a right angle. Make any necessary adjustments by raising or lowering your work surface or chair. If your work surface cannot be lowered or raised to accommodate your elbow height, you can raise your chair and use a footrest. The footrest should be large enough for both feet. If necessary, a keyboard tray can be used to bring the keyboard and mouse to elbow height. A keyboard tray should be height and tilt adjustable, have room for both the keyboard and mouse, and should not compromise legroom. 

Materials used frequently should be located within easy reach (a good way to arrange work materials is in a semicircle shape). By keeping materials you do not use frequently out of reach, you will have to get out of your chair for them.  This will promote blood circulation and reduce overall discomfort.