You need to identify your strengths and weaknesses when it comes to the test. Personally, I am a horrible standardized test taker and I get really bad anxiety. Therefore, I knew I had to practice practice practice.
I spent about 3 months studying for it. For the vocab, I got Kaplan's vocab book and pretty much memorized as many vocab words as possible. For the quant section, I found Barron's GRE book extremely helpful. As for the writing section, I went to ETS' website and I printed out every single issue essay possibility and grouped the similar ones together. Then I would practice writing out essays and post them to forums for other people to grade. In total, I spent about $80 on practice material.
The number 1 tip I would give is to practice taking the test. My library offers free GRE tests to practice on, plus Barrons gives you CD-ROMS with 2 free tests to practice on.
Most importantly, relax and breath. It's not that difficult. If you did well on your SAT, I suspect you have the test taking skills to do stellar on the GRE.