Take as many practice tests as you can

Ideally under test conditions. Particularly if you are using the Official Guide for GMAT Review or one of our GMAT Paper Tests, try to replicate the test day experience. For example, try a practice test with your copy of the Official Guide for GMAT Review placed upright like a computer monitor and use a sheet of scrap paper rather than writing on the test questions, as you will not be able to do this on test day.