I take extreme pleasure in rubbing it in the face of a colleague who has the 1.2 petrol, and gets only 12!!
BTW, the car is on an exclusive diet of Power, and I check the air pressures every time I fill up ... or two weeks, whichever is earlier.
I think that does help a lot.
I try to avoid unnecessary braking, and am in a habit of switching off at traffic lights.
But I think a very under-rated factor is the air pressure ... I know it can be a hassle to check the pressure every time ... but it is paying great dividends. Only, you should go to the same guy every time, (and his pressure meter should be accurate!!) one of those places where you dial in the air pressure is best. If you check every time at a different place, you will never have the correct pressures in.
I fill the tyres when cold, and fill it to one psi more than the recommended.
Over inflation will give you better mileage, I agree, but the trade-off is in ride and handling, and the tyre noise goes up by a major factor!!