As far as my prep experience goes. Data sufficiency mistakes fall broadly into two categories:
* Topic difficulty, unknown concept
* Strategy
The former refers to certain topics that one is sometimes weak in or unfamiliar with and the latter refers to the "approach" that one uses to solve DS questions within the broader 2 minute requirement.
If you are struggling with the approach aspect, i would suggest that you go over on the MGMAT website and check out Thursdays with Ron videos on DS. They are awesome as far as strategy, paraphrasing etc. If you are struggling with the concepts then you would have to hit the basic prep books again for more practice. For a source of practice questions try the OG DS Section (includes DS questions of all difficulty level) or buy a dedicated DS book. I recommend the Veritas Prep DS book (10 bucks used on Amazon, or 22 bucks from VP along with 5 CAT's), it includes 116 (iirc) questions from the real basic ones to a few dozen moderate to advanced questions.
CR is a bit tricky, you have to REVIEW concepts and practice questions until you develop good accuracy. One good thing is that once you improve your CR skill you will most likely also see improvement in your RC.
"When you want to succeed as bad as you want to breathe, then you'll be successful." - Eric Thomas
