i was pretty much going to say what this guy said
"typically i like to use some rope and a good ole telephone pole (~0$) but a rubber mallet ( 5$) or a body shop (<50$) would be just as good"
except, i was going to say to take it off, lay it on the ground on something that won't scratch, and bang the shit out of it with a sledge-hammer in a systematic and well controlled manner so as not to over-bang the shit out of it...
this may sound like i'm being a smart ass, but... i'm really not.. that's what they'll do at a body shop. and they'll charge you for a brand new one most likely to! the only thing new on the bumper would be the armoural they put on it.
sad but true... you gotta watch out for that kind of crap.