There are an almost unlimited number of ways to save gas, so let's get started. We'll assume you have already decided to not buy a vehicle which uses alternative fuels and work from there, starting with the intent to use the vehicle you already own.
Driving Style
There is a lot of potential saving in just driving with saving gas in mind. Shifting into neutral (for both manual and automatic) when you know you're about to stop will keep RPMs down, and when the engine isn't rotating as quickly, you aren't using as much fuel.
Keep Your Vehicle Maintained
Some basic things like changing your oil and air filter when needed can really help. When your car loses power because of overlooking these things, it has to be driven harder to achieve the same speed.
Improve Your Engine's Performance
Adding power in order to save fuel may seem counter-intuitive, but doing in the right ways and in combonation with a change in your driving style can show quite an improvement. As long as the power doesn't come from forced induction (like superchargers) or an increase in displacement, the power will only be freeing up wasted potential, and means you can either shift earlier in a manual (to keep RPMs lower) or drive conservatively in an automatic (also to keep RPMs low). Examples of ways to add power without using more air/fuel to get it include installing a free-flowing exhaust system, performance air filter, or porting the intake manifold. In a similar way, you can also reduce losses of power in the drivetrain by installing a smaller drive pulley on the engine, using a lighter-weight driveshaft, or using lightweight wheels. If you have an older car without fuel injection and a fuel injection upgrade kit is available, buying installing one is highly recommended.
Reduce Your Vehicle's Weight
Stripping out every unimportant part probably doesn't sound like a good idea, but you can, however, still reduce some weight by not carrying around stuff you don't need, especially trash. If you aren't too concerned with your vehicle's appearance, you can strip out parts you don't need, ranging from a few sound-proofing parts to completely removing everything except the dashboard and driver seat.
Installing Larger Diameter Tires
If there is room in the wheel wells of your vehicle to install larger tires, doing so can trade off acelleration for fuel economy (and top speed). An example of how this works is, assuming you have a driving routine, a stretch of road 10 miles long that you always drive 50 MPH on keeps your engine running (in a particular gear) at 2,000 RPM; driving down that same stretch of road, all other conditions being the same, will result in a lower engine RPM after larger tires are installed, and an engine that has fewer combustions per minute is using less fuel.
Swap Engines
This is a more complicated and clostly means of saving fuel, but swapping out your current engine for an engine which uses less fuel (usually an engine with a smaller displacement) is a good place to start.
Adding Acetone to your Fuel
In theory acetone will aid in the vaporization of gasoline, which will improve combustion efficency. However, you need a fuel-injected car with working O2 Sensors that can detect the new improvement in order to gain from this. Additionally, the corrosive nature of acetone may eat away at certain types of rubber hoses and gaskets, which could lead to costly repairs. Putting a very small amount in per 10 gallons of fuel should dillute the acetone enough to eliminate this effect. Cars Simplified does not recommend using acetone; when testing was conducted, no noticable effects were noted. Cars Simplified takes no responsibility for vehicle damage due to use of acetone or similar chemicals. Please also note that acetone removes paint, and spilling it on your car can cause aesthetic harm. Please follow directions on the bottle.