The best way to improve your vocabulary is simply to read more. By seeing words in a meaningful context, you are more likely to retain their meaning than simply memorizing from a list. Many contemporary works of fiction for young adults feature words commonly found on the SAT; consider popular works such as the "Harry Potter" or "Twilight" series to make learning vocabulary fun. Newspapers like the New York Time" are also a good source of college-level vocabulary, and as a bonus, you will enhance your knowledge of current events, which can help you on the SAT essay and in college interviews.
When you encounter words you don't know in books, newspapers, movies or on TV, write them down in a notebook. Try to guess the meaning of each word, then look up the actual definition in the dictionary and write it down. Keep in mind that many college-level words have multiple meanings and forms, and make note of these in your journal. Review your journal periodically, especially before test day, to keep the words fresh in your mind.
Because the SAT uses many vocabulary words again and again, working with practice tests can help you to build your vocabulary knowledge in addition to improving your test-taking skills. Take note of words that you miss or don't know on each practice test and make a list of them. Look up each word and write down its definition. To help the word sink in more, write a sentence using the word, or use multiple words in a short story.
There are many books and flash-card packages that feature the top SAT vocabulary words, but simply memorizing a list of words and definitions is not the best way to retain this knowledge. Mnemonics are images, rhymes or associations that help you to remember definitions of words in a more meaningful way. Try envisioning an exaggerated image that represents a word every time you say it, or associating the sounds of a word with its meaning. For example, to remember that "buttress" means support, you might think to yourself, "A big butt gives you extra support, like a buttress," and so on.