Stick shift or automatic? Either way, and assuming that you take the advice as already given, the best and cheapest way to improve fuel efficiency is to drive gently and be aware. The safest drivers are also the most economical. A good driver plans ahead, chooses the correct lanes, plans the route, anticipates needing to stop and therefore does'nt need to brake suddenly. There are many books on correct driving techniques. Stick shift cars are alsol more economical than autos, when driven correctly. Choose alternatives to the car where possible, a short walk to the local store saves energy and provides exercise.
Regular servicing, re-mapping for economy (check with your insurance), tyre pressure and condition, suspension condition, can all have a hige impact on economy.
Altering driving style can yield savings of up to 20%,
Re-mapping can yield savings of up to 15%, more in diesel cars than petrol.
If you want to carry out alterations, all details of your car model and what you hope to achieve will be important.
For petrol cars, the most efficient engine speed is about 2500 revs. This means that if you choose a car that has a peak torque at this speed, at times when you require additional work (asking more of your car) from your car (climbing a hill) you should aim for this engine speed. going higher will reduce effiency, as will slower engine speeds. However, lower speeds when no work is needed (descending a hill) will use far less fuel, so allow lower engine speeds when no work is being done. The most efficient speed for diesel cars is about 1600 revs.
For regulating speed, use the engine breaking. For stopping use the brakes. For slowing down, the engine braking will be more economical, but be aware that engine braking has no lights, always gently use the brakes if only to illuminate the brake lights for vehicles behind.
Hope that this helps.