I forgot this one earlier - air conditioning. The AC really knocks down the mileage - although at high speed it is better than opening a window. But at low speed - cruising in the city at 35 or under - it is probably better to turn the AC off and open the windows wide. Besides, if you have a thumpin stereo might as well give the populace exposure to decent music instead of top 40. :-)
And don't forget the defroster turns on the AC. You can override that and turn on defrost then turn off ac. it will take longer to clear the windows of course.