I'll give my $.02 here;
First some background;
I'm a mechanic from MN (currently residing in AZ), and have professionally replaced many a destroyed/rusted exhaust system on many cars while living in MN. I also recently worked as a 'corrosion specialist' at VW of America here at their proving grounds near my home in AZ (probably one of the reasons I was hired), so I know a bit about rust. VW offers a rust/corrosion warranty on their cars, hence their need for a corrosion dept.
In my experience, the best way is prevention. But once rust/corrosion sets in, there are only a few things you can do to get rid of it;
1 - replace it,
2 - take the part and have it dipped somewhere to remove the rust, then treat it with an anti-rust inhibitor
3 - media blast it, then treat it with inhibitor
4 - spray or brush on some inhibitor/neutralizer, then spray it with rustoleum.
At the end of the day, it's probably cheaper to just spray on some hi-temp rustoleum or equivalent made for barbecues, or buy a new exhaust.
But probably a new exhaust (#1) is the quickest way.
Maybe some other have some better idea's...?