It seems I've always had a very slight exhaust leak where the muffler slides onto the header pipe and have never been able to fully seal it just torqueing on the clamp. I knew there was some sort of sealant tape around and finally found this:

The stuff was easy to use and seems to work pretty well. Be advised that it seats using the heat of the exhaust, so you'll smell it for the first several miles you ride when you stop.