There are a number of companies that offer components that can attach to your car's CPU and can make adjustments to increase gas mileage. These can be quite expensive however.
There are some cheaper solutions that can help. Make sure your tires are properly inflated. A lot of people don't realize that their tires are often under-inflated, and this can significantly lower gas mileage.
Also, while it might not always be fun to drive the speed limit, doing so can help save gas. Speed limits on roads have been determined not only for safety, but what will also be most efficient fuel-wise for your vehicle.
I hope this helps :)