I've found use of a dead blow hammer to get a firm bolt loosened does wonders. If you don't know what one is ask somebody at your local hardware store. They are plastic with sand in there and a great tool for busting things loose or seating things (much better than a rubber mallet ime). Less chance of stripping a bolt using one of these.

Also, a heat gun might help loosen it up if it is really bad (or a hair dryer if you don't have a heat gun but that doesn't get as hot).