Change gears prudently when you’re accelerating. Never ‘redline’ the rev counter. 

The faster you go, the more wind resistance you’ll encounter and the more fuel your vehicle will consume just to maintain speed. Driving just 5mph over the speed limit can affect fuel economy by up to 23%. 

Air conditioning puts added strain on the engine and uses fuel to operate, so limit use to particularly hot or cold days. When possible use the fan instead.

A new study shows that replacing a clogged air filter on cars with fuel-injected, computer-controlled gasoline engines does not improve fuel economy but it can improve acceleration time by around 6 to 11 percent. This kind of engine is prevalent on most gasoline cars manufactured from the early 1980s onward.
Tests suggest that replacing a clogged air filter on an older car with a carbureted engine may improve fuel economy 2 to 6 percent under normal replacement conditions or up to 14 percent if the filter is so clogged that it significantly affects drivability. The effect of a clogged air filter on diesel vehicles will be tested in the near future.
Data Sources
Information on the impact of air filter condition on fuel economy is based on a study by Kevin Norman, Shean Huff, and Brian West, Effect of Intake Air Filter Condition on Vehicle Fuel Economy, Oak Ridge National Laboratory, 2009.

If you can travel outside of peak times, do so, you’ll spend less time stuck in traffic and consume less fuel as a result

Instead of coasting when you reach a downward slope, maintain steady engine revs.  This will mean you pick up speed, and if you've then got a hill to climb, it gives you considerable added momentum to help go up it, energy efficiently. 

When you’re not calm, you’re more likely to make errors of judgment.  Fuel efficiency is all about smoothness, and keeping calm is absolutely crucial to achieving best fuel economy results.

When you're faced with a headwind, the engine has to work much harder to cover the same distance because of drag.  The only thing to do is to moderate your speed to ensure that the engine doesn't end up labouring too hard.