You can't just do practice questions. You MUST take timed practice CATs (the computer test) so that you are comfortable with it on test day. On the real GMAT, you can't skip around and you must get your timing down. If you do not do full tests, you may very well do poorly no matter how many times you do practice questions.
Rather than repeatedly doing the same questions, get some good practice tests right away. Download Powerprep
free from mba.com (do these before you do the OG because several questions will overlap), and get the Princeton Review
book, Cracking the GMAT
. It comes with 5 tests that are very similar to the real thing, very realistic.