This is a well known one, but I only recently got around to actually doing it, so I suspect that there are others that also haven't done it yet.
Learn to touch type. The kind of person you probably are if you are reading Less Wrong spends a remarkable fraction of the day typing at a computer. As such, even a small increase in typing speed and skill can save you huge amount of time and effort. And it is not at all hard to learn. This investment of a small amount of time and energy to learn to touch type pays back huge dividends in time saved.
One other point: If you are going to learn to touch type, there is no point whatsoever to doing so in the Qwerty keyboard layout. It is just as easy or easier to learn a better layout (like Dvorak or Colemak), which also will give you a bigger boost to your typing speed and efficiency.
You might want to get yourself this keyboard. It'll decrease your chance of getting RSI, it might be a little faster to type on than a standard keyboard, and it's possible to remap the keys in hardware so you can take your keyboard anywhere and plug it in to anything and have it be configured with your preferred layout. In theory you might be able to train yourself to use Colemak or whatever when your hands were in the contoured position and Qwerty in the flat position, for friends' computers and library computers and such.
(For best results, combine with...
Thus spake Eliezer:
It seems that many here might have outlandish ideas for ways of improving our lives. For instance, a recent post advocated installing really bright lights as a way to boost alertness and productivity. We should not adopt such hacks into our dogma until we're pretty sure they work; however, one way of knowing whether a crazy idea works is to try implementing it, and you may have more ideas than you're planning to implement.
So: please post all such lifehack ideas! Even if you haven't tried them, even if they seem unlikely to work. Post them separately, unless some other way would be more appropriate. If you've tried some idea and it hasn't worked, it would be useful to post that too.