The best release agent I've found is clear plastic packing tape with a bit of carnuaba wax over it.
I used the tape over some rough, cracked spots in a mold and the tape part released better than the smooth parts of the mold.
It does add thickness so you wouldn't want to use it on a bedding job but I have to say most of the bedding jobs I've seen are sorely lacking in masking. You can make clean up easier and get good edge detail if you just mask off the areas where you don't want the bedding compound to go.