I think time management and avoiding stupid mistakes are the biggest issues. Take as many practice tests as you can.
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As for math,
The math is really easy content-wise, but the answers are intentionally chosen to trip you up and, there is the time constraint. Also, you can't go back if you realize you made a mistake. I made a stupid mistake early on the math section when I took it, only to realize the error a couple questions later, and it really messed with my mind and made me to lose concentration. I still got a good score so I'm not complaining, but these testing issues are a real factor.
Basically, even though the math is ludicrously simple (middle school level), don't forgo taking the math practice tests. Sit down and take them in the same style of the test situation (timed, no going back, etc).
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As for verbal,
The verbal section is hugely weighted towards vocabulary. Thankfully, the test makers re-use many words over and over, and people have compiled lists of them which you can memorize. Google gre vocabulary words.
Again, get some practice to get a feel for the test maker's minds. After you take a few practice tests, you can practically predict the wrong answers on some questions just from psychology.
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As for writing,
The key is to write a lot. Seriously. Groups have done studies of scores, and the only thing they could find that the longer the essay the higher the score.
Brainstorm for a couple minutes, and then just write write write. Throw ideas down, flow with it. These are not "real" essays, you don't have to be proud of them. A good way to add length is to just put in concrete examples, even if they don't quite apply to what you're talking about. If you just write a ton of BS that sort of sounds coherent, you can probably get a 4.5/6 or 5/6.