Make your car more efficient:-

It's possible to make big improvements by making minor tweaks to your vehicle.

It's estimated that someone who averages 35 miles per gallon could get 40 mpg by driving better, a near 17% saving.

Thanks to the RAC for help with the efficiency improvement data.
Keep your tyres correctly inflated. Efficiency Improvement: Up to 4%
Lower tyre pressure increases the drag on a car meaning you need more fuel, so regularly check the pressures are correct and your car needs less oomph to keep it moving. Declutter your car. Efficiency Improvement: Up to 2.5%
The lighter your car is, the less effort it needs to make to accelerate.

Therefore, by decluttering, clearing out junk from the boot, and not carrying unnecessary weight, you can make extra savings.
Take your roof rack off that's efficiency Improvement: Up to 2.5%
A roof rack, even unused, adds massive wind resistance to a car, increasing drag and making the engine work harder. So if you don't need it, take it and anything else that's inefficient off. Even closing the windows'll make the car run slightly more efficiently.
Turn off the air con. Efficiency Improvement: Up to 9%
Air conditioning also uses an incredible amount of fuel, so make sure it's turned off unless you really need it. However, if it's really hot, it can become a more effective proposition, as driving with the windows closed is more economical than having them all open, due to the extra drag it causes.

Also, don’t keep the engine running; drive off as soon as you start up and switch off the engine as soon as you reach your destination.
Don't fill it up. Efficiency Improvement: Up 1.5%
Fuel is heavy, so by filling the car up you're adding quite a weight. The less fuel your car has in it, the more efficiently it drives. Thus filling up slightly more often and putting less in (to 1/2 or 3/4 full) will make the car run more efficiently.