I have found heat to be the best way of breaking them loose. If you have access to an oxy/acet. torch, that's the best. Propane and map probably won't get it hot enough. If it's an aluminum head, be very careful with the temp, too much will more or less evaporate the aluminum. An impact works well too. Try to work the bolt back and forth, alternating directions. Don't go crazy or it will break. You want to gently impact a little in each direction. When you see it start to move, continue working the bolt a bit in one direction, then a bit in the opposite direction. Gradually move it a little more each time. It should eventually back out.