1) Use low viscosity engine oil (recommended one), 5W30 API SN, or ILSAC GF-5 full synthetic for better fuel consumption.
2) Replace old air filter with Toyoya genuine air filter.
3) Use stock wheels size 165/70 r14 ( I assume it is Vitz 1.0 L)
4) Get rid of extra stuff from the trunk.
5) Get the throttle body serviced (if required).
6) Replace spark plugs ( if required).
7) check O2 sensor for proper functioning.
8) Maintain proper air pressure in the tyres. (34 psi for 165/70 r14)
9) carry out wheel alignment and balancing (if required).
10) Replace tyres if worn out. ( measure tyre tread).
You should get around 14KM/L fuel average in city and minimum of 18 to 19KM/L on highways. Use HOBC petrol.