Take care
Have your tires properly balanced and keep the vehicle "in alignment" to get the best fuel mileage.
Service your vehicle regularly. Keeping a vehicle well maintained keeps it operating efficiently and reduces fuel usage.

Check your tyre pressures
Check your tyre pressures regularly - make it part of your Sunday routine or something. If your tyre pressures are low, you will be increasing the rolling resistance of the tyre on the road and that will be robbing your fuel efficiency - your gas mileage will be down. So make sure they're up to manufacturer recommended values (at the very least) and watch your mpg get a little better. 

STOP SPEEDING!
Fuel consumption increases significantly over about 90 km/h. At 110 km/h your car uses up to 25 per cent more fuel than it would cruising at 90 km/h. If your car is fitted with cruise control, using it during highway driving will help to maintain a steadier speed, which will save fuel.
Consider a little less braking if you can. If you can see the next set of lights ahead of you are red, don't race up to them and come to a complete stop. Try to moderate your speed a little if you can do it safely.   

Change octane if you can
Too many people drive around with medium or premium gas in their tank when they just don't need to. If your owner's manual says "regular", it means it. 

Use the internet 
Not so much about improving your mpg as saving money; no matter where you live, there will be one or more internet sites that can provide you with petrol prices in your area.

Travel Light
The more weight a vehicle carries, the more fuel it uses..Don't use your car as a mobile store room. Leave heavy items like tools and sports equipment at home when you don't need them on a trip. Even roof racks can add to the drag of your car take it off when you’re not using it.
 
KEEP IT CLEAN
Your air filter is what protects your engine from ingesting all the dust, dirt and crap in the air. If it's doing its job well, it will clog up, much like the bag of a vacuum cleaner. Once it clogs up, your engine has a harder time sucking air through it.
Change your oil and oil filter Whilst you won't see any massive improvement by changing your oil and filter, you're ensuring that your engine is keeping its 'fresh blood'. 
If your fuel injectors have never been done, or you've got more than 60,000 miles on them, consider getting the professionally cleaned. It won't be cheap but it's cheaper than a new car (by a huge margin) and it will help your mpg

SPARKS
Spark plugs work in an incredibly hostile environment. If you've got more than 30,000 miles on yours, change them. Fresh plugs that aren't covered in carbon desposits will certainly help you in your quest to become a fuel miser. 
 
Turn it off
Restarting your car takes less fuel than idling.
Most cars don't need to be "warmed up" by idling before setting off. This simply wastes fuel. Start your car when you are ready to go. Once on the road, minimise fuel wasted in idling by stopping the engine whenever your car is stopped or held up for an extended period of time. By having the engine switched off, even for a short period, you will save more fuel than is lost from the burst of fuel involved in restarting the engine.

THE RIGHT GEAR
Driving in a gear lower than you need wastes fuel, and letting the engine labour in top gear on hills and corners is also wasteful. In a manual vehicle, change up gears as soon as the car is comfortable with the higher gear but without accelerating harder than necessary.
Automatic transmissions will shift up more quickly and smoothly if you ease back slightly on the accelerator once the car gathers momentum. Avoid the use of power options which drop the car into a lower gear and therefore use more fuel.

ITS A CAR NOT A FREEZER
Airconditioners can use about 10 per cent extra fuel when operating. However, at speeds of over 80 km/h, use of airconditioning is better for fuel consumption than an open window as this creates aerodynamic drag. If it is hotter inside your car than outside when you start a trip, drive with the windows down for a few minutes to help cool the car before starting the airconditioning.

Hang with the trucks
Ever notice how, in bad traffic jams, cars seem to constantly speed up and slow down, while trucks tend to roll along at the same leisurely pace? A constant speed keeps shifting to a minimum -- important to those who have to wrangle with those ten-speed truck transmissions -- but it also aids economy, as it takes much more fuel to get a vehicle moving than it does to keep it moving. Rolling with the big rigs saves fuel (and aggravation)