GMAT Strategy

I just took the GMAT today and got a 600 which is about 100 points lower than what I had been scoring on the practice tests. For the past month and a half I had been scoring in the 680-720 range on my practice exams. I felt very relaxed during the test and the material didn't seem to differ from that of my studies. What gives? From my practice exams and studies I feel I have a high score in me, but I am a bit discouraged with today's outcome. I plan on retaking the test after the new year. Any tips/advice would be much appreciated.

What was the breakdown in quant /verbal, and what kind of practice tests had you taken? (Kaplan, M-GMAT, etc)