One of the first things people learn about the SAT is that you get penalized for guessing. While this is true, it ignores a very important fact: guessing only works against you if you have absolutely no idea what the answer is. As long as you can eliminate at least one answer choice, guessing works—albeit slightly—in your favor. And the more answers you can eliminate, the more guessing will work in your favor. On the other hand, not guessing on questions in which you can clearly eliminate a few answers can make a 50-60 point difference on your final score.
Another important fact: there are very few questions in which you are truly clueless on. The reason I mention this is many students refuse to guess, claiming that they have no idea. When I prod them, they tell me that there is no way a few answer choices could possibly be correct. Yet when they say they have no idea, they mean they have no idea what the answer is. So think of the SAT not as getting the right answer, but eliminating the wrong ones.
Finally, many students claim that, when guessing between two answer choices, they always choose the wrong one. In reality, students tend to be very selective in their memories, remembering when they guess incorrectly, not when they guess correctly. So always guess if you can narrow it down to two answer choices.