What LEVE said.
Take it off, check it for flat with a stright edge, and use a new, good quality exhaust gasket, metal side to the head.
New bolts are always a good idea, and keep retorqueing them everyday you dirve it for a week or so.
They should stay tight after that.
If you are running headers, pick a header that has a thick exhaust flange. The thicker the better and the less they will warp and leak.