The easiest I've found is to not have the exhaust mounted on the end until you get it on the engine. Lube the exhaust port on the engine with some light oil/wd40/3-in-1/aro (one of those), slip the o-ring into the header, work it on the exhaust port in a twisting/rocking fashion. Once on, hook the spring on the tank side first, then with needle nose, grab the other end, pull and hook it to the other side of the header. I use two springs for extra security and a good seal. Once that's done, I mount the exhaust hanger wherever it needs to be on the other end.
By the way, what engine is it and what brand of o-ring are you using?