GMAT superia is compiling an online repository of - easy to understand tips for math/verbal/essay/IR.  Please copy them into your notebook and revise them regularly.

MATH tips:http://www.semanticslearning.com/GMAT-math-tips.asp
Verbal tips: http://www.semanticslearning.com/GMAT-verbal-tips.asp
Essay and IR Tips: http://www.semanticslearning.com/gmatessay-and-IR-tips.asp