Hi all
I've had the same problem recently - i.e. not the best retention of information.
I've definitely found doing the drills is very helpful at the end of each chapter, as well as going back and really trying to understand the concepts you got wrong
in each drill.
Then, when you do the OG questions, I thoroughly recommend buying the
MGMAT Official Guide Companion - It has answers for ALL the official guide questions.
You can either get it as an online version or I think it gets released as a physical version in a month or two's time (i.e. July/August 2010). The Manhattan guys have gone through and explained the answer to every question in the OG and any alternative ways of solving it. Before I got this, I had no idea how some of the questions were to be solved properly - the OG answers were quite bad in some cases.
Also another method for retention:
Flash cards! You know all the stuff they say to MEMORIZE in the
MGMAT books - DO IT! It's definitely a big time saver when it comes to answering questions. Also make flash cards for topics that are new to you or key concepts (e.g. odd x odd = odd etc.)
There are a few free online flash card generators out there that do the job and test you, you can create flash card sets etc.
Also, write notes on all the stuff that is new to you or you're not entirely comfortable with, then do the questions etc. ,then re-do your notes again with the stuff that is still not sinking in and learn them. Repeat this until it all sinks in. That's just my strategy anyway