We don't expect people to go out tomorrow and replace their vehicles with more efficient ones. But that doesn't mean you can't do anything to improve the efficiency of your driving.

See what your mileage is for a couple of average fill-ups. You can know for sure by filling the car up and zeroing the trip odometer then filling up again and dividing the mileage on the trip odometer by the number of gallons you put in. Do this a couple times to get a good idea of what your baseline is.

Better driving means performing any regular maintenance on your car (don't forget to check tire pressures), removing any heavy unnecessary objects from the vehicle (still have the bags of sand in there from winter?), and accelerating smoothly. It also involves cutting your speed on the highway a bit. When driving around town, don't accelerate to a red light (the more you use your brake, the more energy you are wasting). Other techniques include using your A/C less, planning your trips to minimize stops, and turning off the engine instead of idling (new cars are very efficient at starting up, here's a good article on it).

Continue to track mileage to see what the changes are doing to your mileage.

Make the behaviors (and mileage improvements) permanent. Continue to try get better and consistently raise your mileage. This is an ongoing thing, but once you feel like you've made a significant improvement, you can check off the guide.