There are several things you can do that don't cost very much at all.
1. Check the tire inflation each month. Underinflated tires create more rolling resistance and overheat as well.
2. Take all of the junk out of it. The more mass you move the more fuel it takes. So remove tool boxes, sports equipment or anything else piling up in there.
3. Accelerate at half the rate you normally would. In other words, go from 0-30 mph in ten seconds instead of five. Slower acceleration does help.
4. Keep the speed below 65 MPH. Higher speed does waste fuel.
5. Create a driving plan the reduces the amount of left turns you have to make and combine trips. I know this sounds silly, but the major carriers such as FedEx and UPS use software to plot routes that minimize left hand turns. The more rights you can make, the less time the driver is stuck sitting at a red light getting 0 mpg.
6. Get creative. Do you really have to drive three blocks to the grocery store or can you take the 10 minutes to walk it? I have cut down my driving to a point where I can actually go a whole month without refilling my Mountaineer. It sucks, but I am out to prove a point that it can be done. And I have lost eight pounds between the added walking and cycling.