I've taken the SAT twice and done multiple Princeton Review prep courses for it (I want to go to a good college). First of all, if you can eliminate one answer choice, then the odds are in your favor if you guess. Remember that the questions go from easiest to hardest, and the hard ones don't get you more points. Slow down, and make sure you don't make careless mistakes. Focus on your strengths; if you're great at algebra but stink at geometry, answer the algebra questions first. Don't read the whole long reading sections, just skim them and go back to answer the questions.
There's a lot more I can say, but seriously, I spent months studying before each SAT I took. You can't start studying the night before the test and expect to improve majorly. In my case, the study paid off. I was accepted to my first choice college with a Presidential Scholarship!