Same story here, scored a 400 on the GMAT and got myself up to the mid 500's on practice tests. I think the key here is to be very solid in the basics, and I mean SOLID! After that, you practice the mid to advanced concepts from several sources and then learn how the test tries to trick you. After that you should be fine. That is my plan to conquer this beast
A good source for quant might be that math bible which is located on
https://www.gmathacks.com. Heard great things about it.