There's a lot of eco-technology in the works, but refined internal combustion engines and Hybrid-Electric Vehicles (HEV) are what's currently available. With that said, the basics still apply:
Smaller, lighter, more aerodynamic vehicles with efficient 4-cylinder engines will save you money, preserve petroleum and reduce harmful emissions output into the atmosphere. Other basics include proper maintenance and fuel efficient driving habits.
If you don't do anything else to your car, at least keep the air filter clean and the tire pressures correct. It does make a difference.
Here's a good rule of thumb to help conserve fuel while driving; pretend there's a full cup of coffee sitting on top of your vehicle's dashboard. Now, accelerate and brake gently enough to not spill the coffee. Also, keep highway speeds below 60 mph, and you are officially a fuel efficient driver.
While the refinement of internal combustion, hybrid electric, plug-in electric and alternative fuel technology continues, power from hydrogen fuel cells are what we hope reaches production status in the near future.
Fuel cells, by producing electricity through a chemical process, have the potential to be a super-clean and nearly unlimited source of energy. Consumer practical and affordable Fuel Cell Vehicles (FCVs) may be the breakthrough we're looking for.