The GMAT is a timed test, so practice tests should be taken with the same time limits. Some students spend a long time on difficult questions, leaving them with insufficient time to work on problems that they are more likely to correctly answer. Similarly, breezing through the questions that look easy to save time for harder problems is a good way to make careless errors. Try to work methodically. A time limit for each type of question should be decided on before the final test date.
Test takers often make errors when performing calculations that would be easy in an untimed, stress-free setting. To get around this, test takers should slow themselves down by writing every step. This will reduce the number of careless errors and save time when checking over the problem. Test takers who find that they are working through problems in the practice tests too slowly when they write everything down should devote more practice time to the quantitative section instead of skipping steps.
While it feels good to get something right, test preparation should focus on shoring up weaknesses. Use practice tests to determine weak points and then focus on those types of problems until the next scheduled practice test. This should be done not only by looking at what types of problems were answered incorrectly, but by figuring out exactly why the answer was wrong: Was it a sloppy mistake or a serious misunderstanding? As you get better at one type of problem, the schedule can then shift to other problems.
Like other admissions tests, studying for the GMAT is a long process, and some test takers get burnt out. Practice tests should not be taken too often, and should not be the primary studying tool. Looking at thousands of question with minimal understanding won't help as much as looking at a few hundred but understanding them deeply. Take a break right before the test: cramming the night before and showing up tired may be the easiest way to lower your score.