ORIGINAL: altavillan
Permatex Ultra Copper silicone gasket maker. Almost any automotive parts store will have it. Don't use a gasket, just the silicone.
I would just add to that to be sure the mating surfaces are clean and flat before assembly. I usually clean the mating surfaces with something like acetone before applying the sealer. You only need a very light coat of sealer. Gaskets just go to pieces over time and allow the muffler to loosen.