1)Just Stay Home. PetMom beat me to it.
2)Plan your trips in a circle-about format if you can - no unneccesary criss-crossing town or backtracking.
3)Coasting is good - as long as you are in gear. Out of gear coasting (neutral) is said to be NOT SAFE.
4)Take all unnecessary stuff (weight) out of your vehicle.
5)If you're hauling something make sure you create little drag. Meaning if you can, put the stuff inside the vehicle, or if not, make sure it's down low, and/or not squared off on the front so as to create wind-resistance. Think scooped & streamlined packing.
6)Same theory as #5 - keep the windows up if you can.
7)THEY* say that driving without the air-conditioner running is better on gas mileage. (This is one cool sister over here and I won't be trying this out! I keep my AC on as much as possible - in the car & at home! *THEY want to be hot?
I say let them eat cake! Not me, baby! I will save & be kind to the planet elsewhere.)
8)If possible - group your errands/driving into one or two days a week.