The most important tip:
Miss an early GMAT question = average score.
During the exam, the most critical questions are the earliest. So, when you miss an early problem, they kick it down to a much simpler level, which corresponds to a much lower score. Even if you get the next few simple gmat questions right, you will not return to the 650-plus range.
Computer Adaptive Test
The exam is administered at a test center by the GMAT-CAT software, which does a rough search for your most likely verbal and math ranges early in the test. The questions begin at a moderate level and either increase or decrease in difficulty, depending on whether they are answered correctly or not. Then, as the examination progresses, the difficulty is adjusted in increasingly smaller increments as the system zeros in on your "true" score.
Take your time early to raise your GMAT.
On the math section, for example, you solve 37 problems in 75 minutes (two minutes per problem), but the typical problem can be solved in 1:35, giving you some extra time, so don't rush through the early ones too quickly. (but we are getting ahead of ourselves here, because...)
Nothing will increase your score if you do not understand the questions!
Through proper test preparation, you should be able to recognize and automatically react to any of the early, moderate-level gmat questions.
[ Next tip: should you take a GMAT prep course? ]