I feel your pain; I started a few weeks ago with a 560 on PR's CAT.
I took a GMATPrep last week and got a 640 (!!!!!)
I took a Veritas CAT today and got a 560... so it can be frustrating!
My best advice to you is to get
the official guide ASAP; it is much more difficult but it will help you (force you!) to try and think more logically and analyze everything better. The PR and Kaplan books are good for tips and strategies... but spend alot of time reading the
OG's section on arithmetic and properties of numbers etc. In all my years of highschool and college I was never taught to think of division as "y=xq+r"
And spend some time going over the problems you get wrong! Half the time I seem to make stupid mistakes and answer the wrong question or only answer it halfway. Spend time really contemplating how you fouled up the tough problems -- and hopefully you'll run into them again later when you have mastered it.