Fill it, shut it:
It is always advisable to have full tank. Try and fill up above 10-15 litres every time you visit the petrol pump. It’s mileage-effective as well as a saviour in emergencies.
The air-pressure in the tyres has a lot of influence on the mileage. Get it checked regularly.
One of the biggest factors in bringing down mileage is the AC. Use it judiciously and switch if off ocassionally while driving. Also check the coolant.
Check brake oil regularly. Keep carburettor clean.
Keep in mind:
Use high octane petrol once in a while. Move to the fifth gear at 40kms/hour speed. But don’t drag it if the speed falls. Come down to a lower gear.