GMAT Strategy

I have studied for the gmat for months, did all the questions in the official guide, and took the gmat focus diagnostic exams online. When I felt that I had a strong handle on the material and concepts and then took three practice exams with gmat prep software. I got a 630, a 650 and 690 on the practice tests. I took the exam today and got a 590. I felt really good about the test while I was taking it and thought I was doing well, and clearly I was not. I would love to hear some advice on what I can do differently as I prepare to take the test again. I wonder if I gave myself a false sense of security by doing progressively better on the practice exams and got a little cavalier when taking the real exam. At any rate - if anyone has thoughts or suggestions, please let me know.