I think the Official Guides and their supplements are the most important, because they contain tons of problems from actual GMAT material.

Personally I use the MGMAT quant books to study. They're very well written and go in depth about every subject you're going to find on the test, including specific GMAT wording.

Besides that, people post some very challenging problems similar to those you'll find on the exam in this forum, and it's very helpful to go through them and read different explanations and ways to approach them from some of the members, especially the many GMAT tutors who take part in posting detailed explanations, again including some about specific GMAT wording.
Some very interesting articles on here as well.

The Kaplan GMAT course has a math intensive component that includes word problem sections. They also publish a GMAT math workbook that focuses on the types of problems you will see on the actual test. Be sure to analyze not just which answers you missed, but why. The test anticipates errors that people are likely to make and incorporates those into the wrong answers.

When you complete a practice session, go over the explanations for the problems you missed and try to determine whether you are consistently making certain types of errors (exponents, signs, etc.). By finding a trend, rather than solving a single problem, you can improve your results by several problems at a time. Also, don't forget to go over those questions you were unsure of - even if you got them right. Too many times I see students who say "every time I guess I get it wrong!" Untrue. The reason is they only go over the ones on which they guessed AND missed. Be aware of those problems you were unsure of and review them whether they were right or wrong. Otherwise you will have to guess again when you come to that topic next time and you will not always be lucky.