When you spend a zillion hours a day being a web designer, your eyes start to ache with alarming regularity. My secret tip is that when I'm working late at night on the computer, I'll invert the colors on my screen. You can't use it while doing anything color sensitive (the photos will be negatives), but for answering emails and catching up on Facebook status', it's amazing.
Control Option Command + 8 = et voila! No more eye strain.
Note: The above shortcut is for Mac users. If you're on a PC, try this.