Just thought I'd share my approach to the GMAT. Took it the week before Christmas and got my official score report last week. I could not believe it when I saw the unofficial score on the screen. I thought for sure I had done horribly because the questions seemed to easy...but alas!
My two primary sources of studying were the Official GMAT Guide 11th Edition and the GMAT Prep software. I also downloaded the practice tests from 800score.com.
I started prepping several months before the test. I completed every single question in the Official Guide, carefully reviewing any areas and mistakes that I made. I purposefully saved my GMAT Prep tests until the last week or so before my exam. I got 750/760 on them (I took each of the 2 tests twice since the questions recycle to a large degree). I also downloaded the 800score.com tests -- but I found these not to be very useful or reflective of what the actual GMAT looked like.
In the end -- my biggest piece of advice is to give yourself several months of prep. I think it really helped me. The last week or so I reviewed a couple of key things that I wanted to make sure I remembered (e.g. combinations and permutations) and went from there.
All the best to those taking it!