The only REAL way to increase you MPG is to switch from lead to iron in your right foot.

Seriously, take it easy on the throttle and your fuel tank will keep itself full longer. I've been in Denver for the least two years and I average 21mpg. That's because I'm always romping around town or doing 80mph down the highway. If I could get myself to shift at 2000rpm insted of 3000rpm and keep to the 55mph limit, I'm sure I could bump up that number to at least 25mpg.