A simple concept - To save gas, just take your foot off the gas pedal.
On my way to NYC from Detroit yesterday (I moved to NYC), I got bored and decided to see if I could get 400 miles on a tank of gas (15.8 gallons). Normally it goes about 335, but that's at 85 mph, driving sideways, and generally being a hoon. So I went 55 mph for the entire tank. It was murder, all these trucks on I-80 were blazing by me, and I had to keep my lazy eye on the rear view mirror at all times.
The car has never returned better than 24 mpg, but, wait for it...
13.2 gallons, 405.6 miles --> 30.73 mpg
30.7 mpg!!! Can you believe it? Of course, mine is a car with particular aerodynamic issues, but what a huge difference.