Let me take a moment to jump in and make a note about Roxanne's comment. I'm going to very gently disagree with her about the benefits of keeping a score your don't think will be good (sorry Roxanne!).
First, when you keep a low score, it's on your record for five years, so it doesn't go away any time soon. Second, every law school admissions officer will see that score--so that number will always be in their minds, even if you get a higher score next time. And that lower score could raise doubts. Third, once you have two or more scores, an average score appears. Schools don't use the average score like they used to (it used to be the number everyone uses, now schools mostly look at the higher score), but the fact remains: an average of all your scores will appear on your record for the next five years, and everyone will see it. Fourth, I've talked to numerous admissions officers about multiple score candidates, and never have I heard one express the idea that an initial low score will make a second high score look good. They all uniformly agree that a single, good score is the optimal admission approach because it means the committee doesn't have to think about anything else in regard to your LSAT score. Finally, if you know you didn't do well, then you already know what you are up against--you don't need an official score to prove that point.
The time to keep your score is when you know you did well, or you aren't sure you did badly. If you know for certain you didn't do well--as Eli knows here--typically the best advice is to cancel.
Thanks, and sorry to disagree with you Roxanne. No harm intended!