Like P1000 commented, you probably have your wording wrong. You want to decrease cosumption.
I average 8l/100km almost pure city driving. On my vacation my trip to the coast I got >1000km on a tank just driving at the national limit. Driving hard I don't think I could get it above 9l/100km...
This may not sound as great as the specs, but is sure as hell alot better than the 14l/100km I got with my petrol audi doing the same route. And don't even compare to small hatch backs, they are much lighter. The beemer also has a little bit more losses than those small FWD cars.
As for what to do, there are alot of threads on this forum giving the same advice.
- Drive slower, wind resistance increases exponentially with speed, so driving at 120km/h instead of 130km/h helps. I can't get myself to do this though.
- Anticipate traffic. Everytime you brake, you convert energy into heat, and this is bad.
- Stay in the economical RPM band, diesels are least economical at higher RPM.
- Lose some weight. Don't carry around garbage in your boot that you don't need. Useless weight wastes energy.
- Keep your tyres inflated at optimal pressure.
- If you are seriously pedantic, open windows and turn off aircon while driving slow. Close windows and activate aircon when driving fast.
EDIT:
I do not do any of these things. I have a fun car and I want to enjoy it, even if I have to fill up more. Its worth it.