I always make sure I have good quality gaskets, also make sure that they haven't been sitting on a shelf for years or hanging on a nail on the wall so they get all distorted and dried out. As far as a sealant goes I use a small amount of either #2 or black RTV in corners where parts meet, but as far as the gasket surface goes I've found that if you have a good quality gasket it will seal and fill most slight distortions. I also use 3M weatherstrip adhesive to glue gaskets in place, esp valve cover gaskets that come on and off for adjustments. On cylinder head gaskets I have used aluminum paint, or high tack gasket spray if its a metal type gasket, if its a composition type gasket usually no added sealer works the best.
