You don't have to get a solar car, but there are lots of options today which include, cars that run on Ethanol, CNG, and Hydrogen. There are also hybrids that run of gasoline but converts your braking energy to electricity which can be used later to drive your car. Check out this site from EPA on green vehicles.
Avoid carrying extra weight, clean out your car, every pound / kg of weight causes you time to accelerate and increase the wear on your tires to brake. Extra items placed outside the car will also increase drag especially at higher speeds, try to keep your car smooth, clean and if possible waxed.
Studies have shown that weaving in and out of traffic, causes you to travel further than moving in a straight line, causes wear on your brakes and tires and ultimately increases fuel consumption. Instead try to drive as smoothly as possible.
If you are driving a manual car, using a lower gear than required will increasing revving and increase your fuel consumption, instead switch to the highest gear quickly without straining your engine.
Using your air-conditioner taps into your engine's power and can increase fuel consumption by more than 20% in city driving. Most of the heat in your car is generated from sunlight entering your car, so try to keep in the shade, park under trees, underground, multi-level car-parks. If you have to park outside in the sun, open the windows / doors while you are packing your car.