Hypermiling aside, the easiest way to improve your gas mileage OTHER THAN going lighter on the pedal is to remove excess weight from your car. I'm sure you're carrying stuff you don't need in there. When I recently cleaned out J's car, I found 40 lbs of books she hasn't needed in a year, and some rocks and various clumps of paper, partially filled water bottles and lots of garbage. After cleaning it and removing junk her MPG went up +2. Not too shabby. My own efforts have boosted my MPG on highway, though most of my driving is short stuff. I suppose if I let my car warm up a few minutes I'd be wasting the warm-up gas time, but gaining in fuel economy once it starts moving. There's always tradeoffs, after all.
One thing that made me smile was
this article because it reminded me of a time when I was driving my car delivering newspapers and I removed my front seat and the lower part of the back seat to hold them all. It was lame having them in the garage, but the car was 50 lbs lighter and when you hit the gas, it took off much faster. You might find yourself impressed with what happens if you pull some dead weight out of your car. Might get a bit more zippy and bit easier on the wallet at the same time.
Naturally, this also leads to thoughts of "what if I converted one of those cars to a ultralight one with replaced parts and a lot fewer fancy amenities? Could I live with it? Would gaining 3-5 mpg be worth it to me? True, you might find a different car is the answer, but when you do the math on the cost difference and the time to pay it off vs the gasoline you can buy for the difference, well its usually embarassing. That's why J is driving a Civic not a Prius. The smartest option is to be a good mechanic in the first place and to lighten up a used car to the point its "not as sold" but not illegal either. You might give up some crash safety but you'll save a lot of gasoline. And it still keeps the rain off.
Eventually I feel certain we'll force the govt to reverse safety requirements so we can hobby up our own cars from motorcycle engines and welded steel tubing. That would cut half the weight and drastically improve fuel economy. Thing is, you really need to be a gearhead to pull this off. And the vehicles need to be 4 wheeled. The best way to get around the traditional requirements is to take a vehicle and gut it, swap the engine, and get it refereed for approval by the DMV. It will need the specific stuff like turn signals and brake lights but those don't have to be super fancy or heavy objects. And if you're content to drive slowly you won't need them anyway. Then again, this could be optimism at work. Deadly, how it creeps up on you.