I never achieved 20km/l for my previous ride. How you did it ? 
1. Take life easy. Don't rush. Rushing builds hypertension and we may end up at the hospital earlier.
2. Switch off the aircon, enjoy the fresh morning air...
3. Drive lots of expressway. Build up a few car lengths of buffer.
4. If it's a short trip, use the built in "kah" or bicycle.
5. Drive like car has no power. Brake like car has lousy brakes.
6. If car behind is impatient and flashes light at you, just pretend you have no money to service the car/buy petrol/etc and wave back with a smile...
7. Tyre pressure should be at least up to what is marked on door post or higher.
It helps if you find it fun to try setting a record tank every time you go to the pump.
Waiting for Nissan to build a Hybrid Latio... or maybe Latio 1.4L turbo diesel with 6-MT... hmm...