WD40 on the joint point of the old mufflers made things easy for me.

 

Updating from a previous comment, here is how my bike is sounding with the cycle shack slipons (with the baffles they come with) and a new intake:

 

I went through the same thing as joe. They might be "slip-ons", but they are not "slip-offs". I tried everything I could think of to loosen the stock mufflers from the pipes, but it was still a pain. However, I love the sound.