Used to get 17/21, but now with 265/70/17 AT tires, I'm getting 15/18.
I don't there is much you can do to dramatically increase your MPG. Even if there were some things, you have to weigh the cost to benefit ratio. Putting in a $1200 exhaust (I'm exaggerating) to get 1MPG extra is not worth it.
The only way I can see you getting better gas mileage is if you do a diesel or 4cyl swap. But, again, how long will that take to re-coup the money spent?
I used to keep a very detailed journal of my MPG (since I commute 100 miles a day). But now, I don't really care. I'll keep an eye on the MPG to make sure everything's functioning ok, but I figure it's too much hassle trying to keep under 70mph on the highway, slow take-off's, blah blah blah. If I drive like I want, (80mph, normal take-off, etc), it's only a few dollar's worth of difference at the pump. I can live with that.