I've heard some guys recommend using some high-temp copper RTV silicone around the flanges before you push it all up there and cinch it down. I haven't tried this, myself, and I suspect that it's possible the stuff would just be blown right out (especially if you don't allow it enough time to fully cure), but it's worth a shot. You'll only be out, what, five bucks for a tube, and you can rub it right off with your thumb if it doesn't work for ya'.
Alternatively, you could probably do the same and maybe have better luck with a tube of that "muffler mender" goop, but it sets up a lot faster, so you'd better be quick about bolting stuff together after you put it around the flanges.
Again, it's not terribly permanent and forever-binding like JB Weld or anything, it's just kind of a clay putty sort of stuff, but it's also worth a shot.
Interesting that you should mention using washers to help push the flanges together. I kinda suspect mine is leaking a bit at the header flanges, although I can't for the life of me figure out where (can't feel a leak by hand or by waving a tissue around to feel for a leak), but I've been about gagging from exhaust fumes for the past year or so ... even after changing from one brand of X-pipe to another! So, I might give that a try for giggles - thanks for the idea!