GMAT Strategy

Hi,
Aside from learning the gmat, I am also facing stamina during the exam. Sometimes I tend to loose concentration in the middle of the exam, do you have any suggestions on how to be consistent during the exam, such as doing excercises daily. This is a major problem by the time I get to verbal....I never experienced this academically, but I have been trying for the gmat since Jan and anxiety really killed my score when I took the test in august. I got only 3 hours of sleep. jitters and ran around crazy before the exam, to my surprise I still scored a 39 in quant and verbal tanked to 25. But this was common in my practice during verbal.....any advice on how to practice and copy the success of practice sessions on to the test.