For improving fuel average, in addition to your above points there are few other factors like:
A properly tuned engine
Clean air filter
Proper and balanced tire pressure
Balanced and aligned wheel
Use of good quality fuel from a reliable fuel station
You see none of the above is too much expensive, its just a routine kind of maintenance. if you keep on doing these timely, gradually mileage may improve.
My 2006, gives me 15~16 without AC in winters withing city, now with full time AC, its giving me 12~13 in city or sometime even lesser. Tuning is due, will go next week, insha Allah it will improve.
Best of luck