As a disclaimer, I have not yet taken the GMAT...but have studied for the past 3 months, score about 740 on PP and about 660 on Kaplan. I'll put in my two cents.
I think the single biggest thing you can do to improve is to take one question at a time. Sounds dumb and simple, but if you make that question all you think about in your mind, not worrying if it is #2 or # 37, or if it is easy or hard, then you will improve your score. Also, when your get your answer, esp. is PS, plug it back in, or get it two different ways - quickly of course. You also need to be at the point where you do not need to consciously think about timing, it should be natural.
Fianlly, AkBrah said something about this earlier, and its sooooo true, when you study, IT IS THE PROBLEMS THAT YOU MISS THAT WILL IMPROVE YOUR SCORE!!! When you miss a problem, in any section, learn why. What good does it do you to study and make the same mistake again and again? NONE!! I keep a notebook - three sections - one for math, one for SC and one for CR. When i get a problem wrong, I write it down in there. This gets added to EVERY time I study. Before I do some SC practice, I review that section in my notebook. Sound crazy?? I dunno, but if I am putting all this time into studying, why not be the BEST. At first I loved doing PS problems - bc I would get 98% of them right. What good does that do??? I didnt want to do SC b/c I would write soooo many wrong sentences in the notebook. but thats the only way to improve.
Also, I do RC at the end of my practice sessions, when my eyes are the most drained and it is the last thing on earth that I would want to do at that time. I figure that way those are the roughest conditions that I will face.
good luck