Don't give up. The GMAT is written to a very beatable formula. I went from a 510 the first time to a 710 on the real thing (which by the way was well below my last practice test).
My advice is learn the rules of the test. The math only covers certain areas, the verbal only has 3 question types (with some nuances). A course like Kaplan
on-line is well-worth the $499 in my opinion because it teaches you how to frame the questions.
This is like anything, wrap your mind around the scope, and the rest will follow.