Once you're going, try to maintain a constant rate of speed. Just as changes of speed in starting and stopping use energy, so too does speed change once you're on the road. Even a moderate change in speed can dramatically affect your gas mileage. While it may seem that using your vehicles automatic mode while on the freeway is the answer to this problem, keep in mind that the automatic function will likely use much more force, or power, to maintain speed in hilly areas. Use cruise control on flat stretches of highway, but change to manual if you're dealing with hills or long climbs. It may take a bit longer to drive gently up a hill, but you'll save more fuel if you control the force yourself rather than allowing the vehicle to go full power to maintain speed. (Also, remember to switch from cruise control to manual when driving in traffic, as this is not only safer, but will also help you control fuel usage.)
Aggressive driving drains your fuel, as the constant changes in speed create excessive needs for fuel. Aggressive driving includes fast starts and stops, quick lane changes, or increasing speed for a short time before slowing again at the next stop or in traffic. All of these things drain energy and will not save you much time.
Running your car without going anywhere wastes gas unnecessarily. Even in cold temperatures, many car manufacturers recommend driving your car slowly to heat the engine rather than letting it idle to warm up. Similarly, sitting in stuck traffic with your vehicle running, or in lines at a drive-up or drive through will also waste fuel. Consider turning off the engine if you know you'll be sitting for some time.
When running errands that require driving through city traffic and multiple stops, develop a plan. This includes combining as many errands as possible in one trip, and choosing a route that takes you to each stop in succession, rather than having to double back. You can also avoid routes that tend to have higher traffic, which means idle time sitting in lines, wasting fuel.
Use Alternate Methods of Transportation
One of the most effective ways to save fuel is by not using it! For nearby errands, or during warmer months, consider walking or biking to your destination. Also, consider carpooling or public transportation when available.