Review basic algebra. More than half of the math questions on the GRE involve manipulating variables. Remember that if you have an equation and you want to simplify it, you have to do the SAME things to BOTH sides. For example how do you solve 3x + 5 = 17? Subtract 5 from both sides: 3x = 12. Divide both sides by 3: x = 4.
Review basic plane geometry. About one third of the math question on the GRE involve lines, angles, triangles, and circles. Complicated questions will combine figures. Since the GRE will not provide you with geometry formulas, you will need to memorize them ahead of time. You should know how to find the areas of triangles and circles, how to calculate circumference, how to find the hypotenuse of a right triangle, and you should know that all the angles inside a triangle add up to 180 degrees.
Review properties of numbers. You will need to know what a prime number is, how even numbers and odd numbers work, how positive numbers and negative numbers work, and how to work with fractions. You should also have an understanding of square roots and exponents.
Review basic arithmetic. There is no calculator allowed on the GRE!!! All the calculations you do will have to be carried out on the scratch paper provided to you. Successful test takers know their times tables and long division. If you want to join their ranks, you will need to be quick on the draw with addition, subtraction, multiplication, and division. Percents appear frequently as well, so review them too.
Once you have reviewed the GRE math content, you should start timing yourself. Get a book of practice problems and set the timer! The GRE gives you 28 or 29 questions in 45 minutes, which works out to be about 1.5 minutes per question. You can take baby steps by giving yourself 2 or 3 minutes per question at first, and then work your way down.