Set aside some time to study for vocabulary. It can be as little as 30-60 minutes each day. It's important to practice everyday so that your build your vocabulary into your long-term memory.
Make a simple flash card (with word on one side and definition on the other side). Flash cards are a great idea because you can put them in different piles as you learn the words. You can find vocab list in any SAT book or even online (http://www.majortests.com/sat/wordlist.php). You can also buy vocab cards so you don't have to make them. My favorite is "English Vocabulary (Sparknotes Study Cards)"
Test yourself on those cards. Make a goal of memorizing 10 words to 50 words a day (depending on how much or how little time you have before you take the SAT). Make sure that you review your vocabulary flash cards every once in awhile, so you won't forget the ones you've already memorized.
Read, read, and read! Reading is a great way to increase vocabulary. However, be mindful of making sure that you read something in high level. Also, SparkNotes have books you can buy called Smart Novels that will have fun stories and will bold all the SAT words and give definitions of those words on the side. You can also read some of these books online at www.sparknotes.com.
Some magazines that will help with vocabulary as well as give you good informations are "Newsweek", "US News and World Reports", "Business Week", "Time", and "Economist". Some newspaper that will help is "New York Times", "Wall Street Journal", and "USA Today". These definitely use higher vocabulary than local newspaper which is a middle school reading level. You can buy them or read some of their articles online in their own website.
Try to use the words that you memorize in your conversation and writing. This is the best way to retain vocabulary. Try thinking about a sentence that you can use your vocab in. Saying it out loud or writing it down will just solidify your understanding and memorization of the words.