what and have your windows open instead? maybe in stop/start traffic, but not whilst up to speed. the a/c becomes more efficient and uses less and less enrgy the longer it has been on especially when in recirc mode - like keeping the fridge door shut)

the biggest single factor is how you drive - change that and you can see dramatic improvements. if you do a lot of motorway miles, drive at 70mph instead of 80mph and you will probably get about 4mpg better.

some people say go into neutral down a hill, but that's also wrong. with the clutch connected, there will be no fuel injected in that situation compared to fuel injected to idle. which is more? you also do not have proper control in neutral.

reduce weight is one of the most significant. if you fill the tank, you are carrying around extra weight, but you have to offset that against the fact that you will stop more often. it depends on what journeys you do.

you also have to offset that on a petrol against the fact that more will evaporate the lower the level in the tank is

obviously tyres pressures is one, but has been discussed already. and remove roof bars etc to get the least drag (same as having the windows closed and using a/c)