Just buy the Official Guide to the GMAT and attempt all the practice questions (this is the book I used). For the quant. (Math) portion, it is more about being familiar with the format of the exam than anything else. The verbal is probably more time consuming for most of us whose first language is not English. Again, remember practice makes perfect! So, practice as many questions as you can. There was something called 1000 questions or something available online a couple of years ago when I took the GMAT. This had 1000 questions for each of the different types of questions on the GMAT.
Good luck!