Don't use your Air Conditioner unless you absolutely have to. Your best mileage is when you drive with it off and windows shut. Your worst mileage is driving high speeds with your widows open. When shutting your windows and turning on your A/C is better for mileage than having the windows open depends on the speed and which windows you have open.
If you are on fast roads, try to drive 50-55 mph. This is the speed your torque converter locks up and you get better efficiency out of your drive train. Slower than that and your torque converter unlocks (45 mph) and above that, you get higher wind resistance and cuts into your gas.