Originally Posted by RAD
As John said, candle wax is best, normally once its freed, Vasaline also does a similar job.
If its currently stuck, then try WD40 as it gets into gaps easier.
I'd stay away from vaseline or WD40, as they are both petroleum compounds (or at least have those compounds), which is notoriously harsh on most types of rubber.
Diving drysuits get a lube of candle wax; there are some silicone zipper lubes that will work well, but are not suggested for use on drysuits (primarily because if the stuff gets into the fabric, it'll be impossible to glue a new zipper on.)
Forgot to add: sometimes cleaning the teeth with a semi-soft nylon brush (think toothbrush) helps quite a bit, too.