Tad's tip is to invert colors on your display. It might look like a photo negative at first, but if you are dealing with a lot of black text on white backgrounds, this can make it easier to see. It also helps if you're working on a plane or in some other public place; it seems like it's harder to parse the inverted screen from other angles so it also gives you a (perceived) bit of privacy. Here's how to do it:
First, you have to enable the option to invert colors in Accessibility settings. A quick way to access that panel is to press Command, Option, F5. When the panel comes up, you can check the box under Display to invert colors:
Accessibility Preference Panel in Yosemite.
If you want to do this on a regular basis, your best bet is to go to Keyboard Shortcuts from that panel, then click on Shortcuts. Under Accessibility, you can enable or modify the keyboard shortcut for inverting colors, and now you can flip back and forth without having to check a box:
Keyboard shortcut preferences.
Now if you need to invert colors for any reason, you can set it up to happen with a simple keyboard shortcut:
Mac Desktop in regular mode.
Mac Desktop in inverted mode.
Even better, it won't do anything weird to what you're already working on; screenshots and other items retain the non-inverted colors (I had to shoot these comparisons with my phone). Thanks Tad!