The best way I have found is to put the flange part in a vise and lean in to it. I have also kicked em with steel toed boots and while that works, I really don't suggest it.
Way back some one else asked this same question. I forget who but some one parked their car on it and then bent it by hand. The pictures were 2 funny.
There are many ways to deal with this and please post what works for you. Here is a search on the subject. Some great information. muffler clearance - Google Search