I didn't have to take the GRE, but my GMAT score was horrid, for me. I went directly from studying and passing all four CPA exams to studying for the GMAT, thinking it would be more of the same. Several hundred hours and tears later, I finally took the test and got in the 83rd percentile, 50th in quant... and decided I was never going to go through that hell again. But it really has nothing to do with IQ or intellegence. I had a 3.97 undergrad GPA and a 3.8 grad GPA first go around, and obtained a CPA before 25. I just don't use that kind of math often and didn't particularly pay attention to math in school, so I was bad at it.
Take it seriously enough, because some programs love to wave their student statistics in everyone's faces. But, don't get discouraged, and don't take it too seriously. They look at other factors too and their scores vary dramaticly. As far as general test prep while working goes, set aside about 1-2 hours a day to study, and don't let anything get in the way of that. Set weekly goals, and find a good test prep program. I know they may look expensive, but the structure can help a lot. Set a date to take the test before you start studying, and don't plan on dragging the studying out too long or you will just lose what you studied at the beginning.