1 - As a rule, go for predictive/preventive rather than breakdown maintenance. My brother's GLi was with me for a while. With him, it gave 8-9 .. with me, easily 13+.
2 - Keep your filters clean.
3 - Get your throttle body serviced.
4 - Engine tuned.
5 - Oil change no more than 4000 km in one go.
- Be light on your pedal and keep a check on your instantaneous consumption. When you do that, you'll know after a while how to maximise your fuel efficiency.