Liquid Wrench (oil) works great. Apply the oil, and tap the bolt gently but repeatedly with a wrench or screwdriver. It set up vibrations that allow the oil to penentrate.
Also, if all else fails, get a dull chisel and make a notch on the periphery of the bolt head. Then use a hammer and the chisel (in the notch) to force the bolt counter-clockwise. This always worked for me when I was a MC mechanic. If this still fails, and if the bolt head is in the open, a "stud remover" could get it out. That is a special socket-like head that closes and grips with teeth when turning counter-clockwise. Last resort-drill & use an "easy out". Since it is already turning a little, most likely it won't break off, just don't try to turn it to quickly, as the friction makes the bolt hot and can cause it to break. Go slow.
