Drive slower on the highway. Going 65 instead of 75 makes a BIG difference. Only accelerate enough and use enough gas to get you to where you can start coasting to a stop.
Of course, drive safely first, don't coast from a mile away.
Take it easy at stoplights. Stoplight racing does a good job of turning gas into brake dust and heat. Keep your tires inflated at a normal pressure. Use the AC on the highway if it's hot, open the windows around town.
The windows up/down doesn't make a huge difference. They did a mythbusters on this where they tested it. I imagine that the speed they did the test on ,the cars, etc., all must have had an effect too.
Once you go above about 60 mph, wind resistance increases dramatically. Power to overcome wind resistance at 100 mph is about 8 times wind resistance at 50 mpg. Could be as low as 5 times or as great as 10 times.
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