Write an article based on this "Plan your trips. Plan your route carefully. Maintain a log over time of how many miles you go (the main odometer) and how much gas you put in (from the gas pump, including fractions)."
Keep lists of needs that will require a trip and try to accomplish multiple objectives with each. This will not increase your fuel mileage (the number of miles your car moves for each gallon of gas), but it will help you drive less (which, in turn, means you use less gas). Take the route with the fewest stops and turns and least traffic. Take highways in preference to city streets when possible. Put it in a spreadsheet.  It will keep you focused, and other methods are inaccurate; you will never know for sure if you're saving fuel, wasting fuel or just seeing errors from gas pumps that stop pumping at different points, or fractions of miles being dropped off your 'trip' odometer when you reset it.