I spent many years without the knowledge that " breaking the Seal " of a hard to remove bolt, screw or plug was best achieved by turning the fitting in the opposite direction. This first attempt has a tendency to break the seal between the head of the fitting and the surface it is pressing against. On numerous occasions this little trick has worked. The important thing to remember is not to try and over do it. Usually time and a good lubricant will creep into the space and as indicated above using heat helps. There is no greater sinking feeling than losing the advantage one has with being patient. Also, be sure to know the thread orientation. Left or right hand thread fittings will create some interesting scenarios in one's mind.