Skills that you should work on would include:
simple calculations done without a calculator. An amazing number of people rely exclusive on calculators and have difficulty doing addition, subtraction, multiplication and division. Do calculations in your head when they are small and easy enough, use note paper otherwise. Just put away the calculator throughout your daily life - until after your GMAT.
Work on using proper grammar. Take notice of your own mistakes and other people's mistakes and get familiar with things like subject-verb agreement, modifiers, and pronoun usage.
Try reading some scientific articles -- I recommend the science section of the New York Times. In fact, read lots of things from the New York Times. You can get a digital subscription for a cheap intro rate and their writing is always grammatically correct, and can help get you ready for reading comp.
If you have more time you can look into things like some math formulas that you may have forgotten, but really the three that I have listed are things that take longer to get back and can really worked on before studying begins.
