|I found this image all over the web but couldn't find an original creator. :(|
In more than three words, here's what I mean. When you're writing a paper for school (or a letter to your grandma, or an acceptance speech for a major award) make it your mission to repeat as few words as possible. If, for example, you would use the word "angry" three times, go back and replace two of them with "chafed" and "irate." If you have occasion to use "big" more than once, use "voluminous," or "hulking," or "colossal." See if you're able to create a document that doesn't repeat any words that aren't conjunctions, articles, prepositions, or pronouns.
It's a challenge to do this well. Not every word a thesaurus will provide is exactly interchangeable in every scenario; you'll need to make sure the word you're trying to use actually fits. For example, it's cool to say a house is "big," or "capacious," but you probably wouldn't call a house "burly." It's advisable, then, to use the thesaurus and dictionary concomitantly. Use the thesaurus for word ideas, and the dictionary to confirm that your choice actually works in context.
One last note: This practice will not only enhance to your vocabulary, but also elevate your writing. The most important writing you do isn't really the SAT essay, but let's have a quick look at how you're actually being evaluated on the SAT essay. What's this? An essay that receives a top score of 6 "exhibits skillful use of language, using a varied, accurate, and apt vocabulary"? Hmm...