Do you feed any frozen food? If so, go buy a tea strainer at Walmart (or any store that might carry one) and thaw your food out in that, running it under hot tap water. That will drain off most of the left-over organics that the frozen food was originally frozen in, and should in turn decrease the amount of surface film on top of the water.
If you want a quick solution, I've heard of using a paper towel, and just placing it on top of your tank (while the filter's off) and then removing it immeadiately. Haven't tried this myself though, since I've never had such a problem.