Thanks for the suggestions, all. Sounds like I should pre-treat with zipper ease or equivalent, and keep the zippers clean of sweat after a ride. A buddy of mine recommended throwing the packs in the washer every once in a while, even though the manufacturers tend to recommend against this.
A roll top hip pack would be great, with just a clip fastener. The zipper pouches on the hip belt get on my nerves - wish they were just snaps or velcro or a clip, because that's just another zipper I need to worry about (and those are the ones that tend to get stuck soonest).