Comb it Kook! Surf shops sell wax combs: little tools that when scraped across old, dirty, foot-funky wax make little grooves that will keep you stuck to your stick. Keep a wax comb in your car or in your baggies or, better yet, use one of your mom's hair combs. She'll love you for it.
Get dirty! A little beach sand can also do the trick. Try to use the more course-grain sand close to the water's edge. Grab a handful and rub it repeatedly across the old wax surface. This acts as the poorman's wax comb and will enhance your stick-to-it-iveness.
In addition to slippery old wax, you may wear bald spots in your wax from where your natural stance connects with your board. In this situation, simply use a little "proactive reconfiguration" which means to dig your nails into a more generous wax area and move it to the slippery waxless section. Better yet...get some wax.
