I know that when it comes to boosting one's vocabulary when preparing for one of the standardized tests, some students memorize long lists of words. Some use flashcards, and others might use mnemonic devices--like associating a word with an image.
That's fine if memorization doesn't bore you, but let's face it. Learning those words by "rote" might help you identify a few on the language section of the SAT, ACT, or GRE, but you'll most likely forget them a week after the test. You also might be someone that hates the practice of memorization.
If you want to improve your vocabulary and really learn new words in context, the best thing is to be a reader, and if you've been reading challenging books throughout high school, that is definitely helpful. But in the short term, try studying from the book 1100 Words You Need to Know. This book teaches you vocabulary inductively. In other words, you're first presented with vocabulary words, then through four steps, you figure out what the words mean in context. If you enjoy language, practicing this way won't even feel like studying. Not only that, there are a number of editions of this book, since it has been revised several times. That means each edition has different words. And if you know about study books, the older editions are truly cheap. The book also includes common expressions and colloquialisms that are good to know. If you're an ESL student, these can be particularly helpful. The various editions of the book also have weekly reviews, and even jokes using your newly learned words.
It's the best book on learning English vocabulary, as far as I'm concerned, and I've examined them all.