The best way it to identify the types of problems GMAT tests and the basic rules that apply to those question types and then look at each question to find out how it fits within that type. This will help as you get to the more difficult questins on the exam because you will be able to see the trick they use and not get confused.

For example - if the problem is geometry then the question is which shape is it about (if it is a mix then how do they relate) what is the main issue tested on that shape and how does this fit in with your current question.

aFter you know exactly what the test is trying to ask then you should do better. Also work under timed conditions as often as you can if you have issues with timing.

Another thought is that if you are making small errors in math you should write everythind down on scratch paper so that you don't make a small mistake in your head.

Best of Luck