IMO, you should just take it once. The GRE is a standardized test that allowes other schools/professors to measure your general intelligence. They have a good idea of the type of student it takes to make it through their graduate programs. For me, if i destroy the math, and do ok on the English, thats all it will take probably to get into a MechEng program.
If you gotta keep taking and retaking the test just to get a decent enough score for grad school, you are only setting yourself up for either a ton of very hard work that could possibly be above your maximum level of understanding, or failure.