I use one of the many varieties (there are a bunch) of Hard As Nails from the bottle with a slimmed down brush for trout flies. I found the brush thing works best for me anyway, if I remove the bristles on each side and leave the ones in the center. Seems to make for more precise application.
For bass/pike/panfish flies, I use LA Colors clear polish from Wal-Mart at 99 cents a bottle. This is because, as everybody knows, bass and pike and panfish are more lowbrow, proletarian type fish and do not deserve the high end polish like trout do..