3gmater:
I just finished my second attempt last Friday (5/30), and received a 760. My first attempt was on 4/21 (5 weeks prior to my second attempt) and was a 620.
It sounds like you are scoring better on your practice exams than you are on the real thing (as I was in the first case). The last two practice exams I took (princeton review and GMAC downloadable one) resulted in a 760 and 730, respectively. Like you, when I saw the 620, my stomach dropped (although nerves were getting to me during my first exam and I found it very difficult to concentrate, as I kept on looking at the clock and becoming flustered).
I felt I knew the concepts well enough, so just focused on the mental aspect and reviewed for 2 weeks prior to my second attempt.
Below are a few tips (of course, just because they worked for me, doesn't mean they will work for you):
-I improved my verbal score significantly by changing the way I approached problems. For all CR questions, I broke down the conclusion and premise and found the gap, whereas in my first attempt I was just looking for the answer that made the most sense). For all SC questions, I looked at the differences among the answer choices, and based on the differences, knew what rule it was testing me on, and based on what I knew about that rule, would select the choice that made the most sense. RC, I would spend a couple minutes reading the passage carefully, and spent about a minute on each question, only referring back to the passage for detailed questions.
-My math score improved simply because I was calm, and not constantly looking at the clock, so I methodically went through each problem and solved it, instead of getting nervous when there was a concept I hadn't studied.
Overall, it sounds like nerves are affecting you. I think your mental state when taking the exam is almost as important as what you have studied. Be confident in what you know, and as long as you're solving most math problems within 2 minutes (on average), I wouldn't look at the clock except for at every 10 question interval. It made a huge difference, as I just focused on doing the problems quick and carefully. Same tip goes for verbal (I divided the total minutes by the number of questions and every 10 questions, I would make sure I wasn't off track). Most test prep experts say (and I think it's true) that the early questions are the most important, and thus the ones you should spend the most time on. This is fine, if you're not aiming for a 700+ score, but if you are (and you're capable of it), then you need to be sure that you don't spend too much time on the earlier questions (a little extra is fine, but not 3 minutes a question on math or 2 1/2 on verbal), otherwise you will need to make up lost time on the latter portion, and if you're prone to getting flustered, trying to make up time by solving questions faster may do you more harm than good.
I wore earplugs during my second attempt, as during the first one I had people sniffling next to me (must have been a cold), but it broke my concentration. I definitely recommend the earplugs.
Also, I recommend doing nothing the day before (it may feel silly that you prep for 2 months, and on what you believe to be the day that you should really buckle down and make sure you haven't missed anything, and on that day you don't do anything.) However, I feel it really helps you maintain your stamina throughout the exam.
The day before my first attempt (which was a Monday), I spent about 4 hours on Sunday just reviewing properties of numbers, basic skills on verbal, etc. but halfway through the exam on Monday morning, I didn't feel like doing any more. This past attempt, I worked all week (including Thursday), and didn't look at anything GMAT related on Thursday night (or at work Thursday). My mind felt very fresh in the morning, and I had the stamina to actively take the exam without becoming fatigued. It seems counter-intuitive, but this last point was a big help to me.
Finally, try to get into your GMAT day routine at least a few days before your attempt (i.e. wake up at the same time, eat similar foods). That way, at least physiologically your body isn't confused. Be confident in your skills, and if you have been doing well on practice exams (I only took princeton review practice tests that came with the "cracking the gmat book", and the ones I downloaded off mba.com), you have nothing to worry about. My last 4 practice exams were 760,730,730, and 760 before I took the real thing and got a 760. I really think those exams were a good indicator of how I would do under circumstances where I control my nerves, and am confident in my abilities.
Anyway, hope that helps, and if you have other questions, let me know.
Good luck!