The best time to think about fuel efficiency is before you buy a car. Generaly, smaller cars will use less gas. And since insurance and taxes are also more of a bother in large cars, it may be wise to buy a little car if you care about the money. Also, among the larger cars, some are better than others. All you have to do is relate to the miles per galon figure that comes with it. Sports cars, by the way, are the best way to make more fuel burn away, and automatic transmission is also a good way of wasting a lot of it.
But those seem too obvious. The interesting thing is how to make a given car which you already have in your garage, go further on every single litre. As a rule, you can improve fuel efficiency by as much as 25% simply by changing your driving style, provided that your car has a manual gearbox.
Here's what you do: Try to change the gears (up) without the engine going to high RPM - certainly not over 2.5k RPM (2500 rounds per minute). I know a lot of people extend first,second and sometimes even third gear more than that because they want to accumulate speed. This may get you to the next traffic light before the car on the other lane, but there's a price to be paid - paid with a currency called petrol.
As long as you change the gears early you're pretty much maximizing the fuel efficiency of your motorcar. One last tip: Stay away from traffic and don't crawl (using the cluctch) too much. In other words, driving in the city is wasteful where fuel is concerned.