I will preface this by noting that I tend to be comfortable even in a pressurized testing environment.
My first practice test was through Princeton Review in May, on which I scored 620. My biggest issue in that practice test was not finishing the quantitative section completely. In the two practice GMAT tests (official ones) I scored 750 and 730, respectively, though I did these in the five days prior to the exam itself, not early on in prep.
During the previous months, I did all the problems I could. There were very few times where I was able to study for 2-3 hours consecutively, so I found it useful to schedule 20-30 minute sessions during my day. Each of these sessions was timed. I was able to get to a point where I was comfortable with about 1 min 45 sec for quantitative questions and about 1 min 30 seconds for verbal (1 min 45 sec for RC). As I look back over my prep materials, a few things stand out:
1) No company can exactly emulate what the GMAT does as well as those who write it. Use their prep materials, not just once, but multiple times. Understand the questions inside out. I found it extremely helpful to understand how they developed answers to these items. I was able to be much more careful during the actual exam as I anticipated distractors.
2) For those who are pushing for a high quantitative score (mine went from roughly 43 to 50 over the course of studying), Manhattan Advanced Quant and the total GMAT eBook were extraordinarily helpful. It is really, really important to think through every question as if it were a data sufficiency question. Ask yourself, what would I need to know to solve this question? Doing this is more important than anything else, in my opinion.
3) My verbal score was high. I owe this to
Manhattan Prep's books on Sentence Correction and Critical Reasoning. Both are outstanding resources. Sentence correction was especially great.
4) Practice pacing. I know I already said this, but pacing is more important than anything else. In a pressure situation, you will default to how you've practiced. If this is within the constraints of the test, you will be surprised at how comfortable you feel on test day.
5) If you know a question is going to be nearly impossible for you to answer, don't waste time. Narrow it down as much as you can, and just move on with it. Have a short memory and don't sweat it.