you guys are really going to start laughing at me now, but I'll admit to yet another bodge I use.
my headers are old and slightly rounded on the ends, from many years of maintanance I expect. it kindof helps getting them on and off, and I've tweaked the mounts to hold them pretty tight by now.
still, they leak around the header, so what I've found works pretty well is aluminum foil duct tape! I take a strip long enough to wrap around the header once, then fold it up longways to make multiple layers (also hiding the sticky side inside itself) the end product being about a 1/4 inch wide and maybe five to eight layers thick. I then flatten this with a roller or spoon or whatever is handy to be super thin.
I loosen the heat dissipating flange, pull it back and wrap the flat strip around the header. I gently stuff this flat bit of aluminum into the crevice around the header into the exhaust hole. it doesn't go all the way in, and leaves about half the width still out. then I slide the flange up against that and gently go round it with a wood dowel and light hammer, tapping the flange against the foil, compressing it into and around the header. then tighten down the flange.
I've redone this a couple of times so far and it actually makes a great seal, and is totally removable as well as not adding a bunch of stress on the head exhaust port! being a light aluminum it molds well to the crevices, doesn't burn up, and really made a difference on my bike. I could hear the exhaust hissing out the headers before I did this, but after it was silent and even in the cold I can't see the puffs of exhaust I used to!
ok, once you stop rolling on the floor, you can tell me what a dolt I am...