Maybe you've seen this video by now of Larry Johnson impersonating Herm Edwards. If not, enjoy.
It's a pretty good representation, although LJ doesn't sell it as well as he could. He's got the sayings down, and the voice partially, but not the mannerisms. That's what separates the great impressions from the good one.
Either way, it's more involved than a Dick Vermeil impression, which consists of just having a good cry.