Allow the car to drift the last 700 yards up to a junction rather than accelerate all the way.
Drive with a maximum 1/3 to 1/2 throttle - if you get the revs right you will not notice much of a loss of power.
Avoid braking and stopping (2 of the biggest fuel wasters) - anticipate the road ahead and roll up to roundabouts. (If you're good you will not touch your brakes on a typical journey!!!)
If its hot open the window rather than turn on the air conditioning, if it's really hot open another window and turn the blowers on, if your steering wheel starts melting then turn on the air conditioning.
Avoid using things that drain the battery - because extra pull occurs on the Alternator requiring more fuel (aircon is very noticable). The big drains are rear window heater, blowers & heaters, headlights.
Inflate your tyres to the correct pressure.
Try to maintain a constant speed - 75 mph uses far more fuel that 65 mph. A distance of 50 miles at 75 mph takes 39.6mins but at 65 it takes 46.2 mins So is 6.6 minutes of your time, the increased risk of being killed, the extra damage to the environment and the extra wear on the engine really worth the extra 25% fuel cost!
Hotter engines are more economical than colder ones - changing the thermostatic control or partially blocking the radiator MAY help on SOME cars. (Run the engine too hot and you will break it - you have been warned.)
Empty the boot of tools you've use and haven't got round to taking out, the camping equipment from last year, and the box of junk you have kicking around - less weight = more fuel economy so if you went on a diet that would help as well.
Keep the car well serviced - change the plugs, oil, filters and check the bearings and ensure that the brakes are not dragging. Fully disengage the handbrake as well when you drive off.
Avoid idling the engine for long periods of time - turn it off at railway crossings etc...
Occasionally put some injector cleaner like redex in - it really does help keep things at peak efficiency and can restore up to 2mpg!
Fuel magnets do not work!!!
Dont' be lazy. Walk to the corner shop - it's only 3 minutes away.
What If There Were No Hypothetical Questions?