Sometimes I put another nut on top of the one all ready on the stuck bolt and give it a few taps with a hammer
It may or may not help. Just remember that it you feel the bolt flex when wrenching on it there is a good chance it's about to break. I have found that one of the most important steps in repairing anything is to know when to walk away from it for a while. A short break can help to relieve some of the stress and gives me a little time to think about other possible solutions
Most know this but this is a good place to add that when replacing the head gasket always make sure the head has a flat surface.
