I was in the same boat, I actually scored a 520 first time I took the GMATprep with Q of 38. Last practice GMATprep I took my Q was up to 47. I too was very lazy in school, eventhough math was my best subject. However, my fundamentals were not very strong.
My recommendation is obviously you have to practice alot. It's one thing to understand the concepts but you need to apply them over and over until it becomes second nature (basically what we didn't do in high school
). What really helped me was getting a study partner. I was lucky enough to have a friend at work that had amazing math skills, he actually took the GMAT and got a 51 in Quant (99th percentile). So just working with him really helped, just to see his approach to problems I was missing. For example, one of the problems regarding decimals had something like 1/9, 12/99...I had no idea any number over 9 basically tells you the number of repeat decimals; i.e. 18/99 = .18181818 and 18/999 = .018018018. This is a trick I would have never figured out.
Get a study buddy and if you can't find one, use these forums. Put in the questions you have issues with and see how others tackle the question.