I teach an SAT prep course, and I tell my students that there are three ways you can get a math problem wrong:
1. You didn't know how to do it
2. You made a mistake while doing it
3. You didn't have time to do it
The key to improvement is understanding your mistakes and correcting them.
Have you been keeping an error log
? If not, go back and look over your mistakes on the practice tests. Try to identify a) exactly error you made and b) what type of error it was.
For the type 1 mistakes the solution is simple: learn the material. Do problems of a similar type to the ones you got wrong (ie. triangles, related rates, word problems, etc) until you can get them right every time. For the types that I struggled with (probablilities mostly), I did a problem of that type 3 times per day for a week. By the time the week was up, I could do it in my sleep. The good news is that most people only have one or two topics that they need to relearn.
If you find that you've made a lot of type 2 mistakes, then you need to work on your accuracy. Most silly mistakes happen because you're moving too fast (you're thinking about step 3 of the problem before you've completed step 1). If you're making mental math errors, then the best thing you can do is force yourself to slow down as you work problems, write everything down and don't skip any steps.
If you're making a lot of type 3 mistakes, then do individual questions in a timed environment. Set your timer for 1 minute and practice moving through problems at that pace. If you want to overacheive, then try to whittle the time down more and more. If you do practice problems in 45 seconds, then knowing you have a full minute on the tests is wonderful.
In summary, the first thing you need to do is figure out why you're making the mistakes that you are and then adjust your prep accordingly. If you figure out what type of error you're making and want more help correcting it, then let me know.
It's also worth noting that, in my experience at least, Kaplan
's math problems were relatively easy, but the penalty for a wrong answer was very harsh. This type of grading scheme is biased towards people who make type 1 mistakes (because the problems simply aren't as hard as they could/should be) and biased against people who make type 2 mistakes (because of the stiff penalty for wrong answers). I suggest that at some point soon you take the official GMAT prep test and see how you do on that.
Hope this helps.