I averaged 35 on both AAMC and Kaplan and ended up with a 30 on the real thing (7/8 exam). PS was 4 points below my practice average, VR was the same and BS was 1 point lower.
I see 2 reasons for this: 1. Bad nerves, especially on PS (the first section, and always my weakest). 2. Extremely hard exam, especially the PS section.
Because of the very hard problems and my nervousness, I kind of froze during the PS and lost a lot of time. Normally, I have to watch my time VERY carefully during this section, as I barely finish on time, but this time I ran out of time with 8-9 questions still to go at the end. DISASTER. I really should have voided, but I didn't realize it then. However, I did know that I probably hadn't done well on PS, so I registered for a retake right after this exam. I did retake on 8/15, and I think it went much better.
So, if there is any lesson to be learned from my story, it is probably DON'T PANIC. Fear can only make things worse for you, so do what you have to do in order to conquer your nerves during the test.
Second of all, know your weaknesses before the exam and drill them relentlessly. And make sure to monitor your time during a section to make sure you're not running into trouble. If time is short, guess on ALL the remaining questions, then double back and do them for real until you run out of time. This way you'll have SOMETHING down for each question, which gives you a chance to earn a point. (Blank questions are always going to be wrong answers, so they really hurt you.)
And if you can tell that you really blew one of the sections (i.e. left a lot of questions blank as I did), it's better to void. You can always retake the test soon after that without having to worry about a low score on your record.