How does driving less give you better gas mileage?

http://www.boston.com/business/personalfinance/gallery/fueleconomymyths/

"At roughly $4 a gallon, many people are desperately trying to squeeze more miles out of each drop. The best ways are usually the most obvious: reduce your load, resist the urge to drive aggressively, and cut the number of trips you make, for starters. However, there are some unusual tips that might surprise you."