I found that playing basketball or table tennis after trading is really helpful to relieve the eyestrain
Light colors are easier on your eyes. More if sitting close to the computer screen. Try to set the background of all your Programs to light colors. Pastel colors come to mind. Avoid bright colors and high contrasts. Lime greens, Fucsias, bright reds, etc are to be avoided as the intensity is too high. Higher contrast and higher brightness are desired for short exposure and larger viewing distances (TV/movies for example), but exactly the opposite to look for when exposed for extended periods of time (PC screens). Contrast & brightness are directly proportional to the distance between your eyes and the screen. The closer you sit the lower the brightness and contrast should be. Try to go as low in brightness and contrast as you can, your eyes will adjust and thank you for it. On the other hand you can set your monitor at a lower resolution (bigger pixel size) so you can sit farther away than where you sit right now however sacrificing some screen real estate. Or you can get new monitors. Large monitors are low in price these days. It will allow you to sit farther without sacrificing any screen real estate. Think of it not only as an expense associated to your trading activity but also as savings in less visits to the eye doctor and delaying/avoiding the use of prescription lenses.
someone posted something on lifehacker about this issue http://www.lifehacker.com.au/tips/2008/04/01/darken_windows_to_reduce_eye_strain-2.html
here's my setup: XP, classic mode. no desktop background, turquoise color? (2) 24" monitors (though 19" i was fine with as well). they're dells, but with the samsung design underneath. they're also the kind which area bunch more expensive because they have a wider viewing angle; i think they're about $700 each? settings 1920*1200. long ago, when i had one of the older monitors, i'd have to stop trading every 30-40 minutes because my eyes would get tired. when i went with the samsung flat screen, everything changed. oh, also, i have them about 3 feet away from me.