There are lot of factors which plays a role in maintaining the fuel economy. I know the following causes of the poor fuel economy:
Sluggish Oxygen sensors
Inaccurate coolant sensor
Engine thermostat damaged
Misfiring
Dirty fuel injectors
Low compression
Low tires
Air filter damaged
All these things leads to the poor fuel economy. And to tackle the same, always take your car for proper servicing in regular interval of times. I know many people who doesn't care to take their cars to the service center as they don't want to spend more money on it. But tryst me, doing that, you will end up spending double the money in long run.