I studied for about a month then took the GMAT. This is what I did:
Went through the OG 11th twice.
Went through the two supplements from the same company(for the verbal and quant. part)
Downloaded the practice tests from mba.com and did a total of like 5.
Took my GMAT and got a 770.
Make sure you understand your mistakes.
If you have major weaknesses in a section then buy another guide and practice some more. For example, I heard that Manhattan SC was really good for sentence correction.
Learn to pay attention because the questions are sometimes a bit tricky and they don't ask what you think they're asking.
Not all CATs are good. I heard Kaplan was ****.