Change your air filter.

Your air filter is what protects your engine from ingesting all the dust, dirt and crap in the air. If it's doing its job well, it will clog up, much like the bag of a vacuum cleaner. Once it clogs up, your engine has a harder time sucking air through it. To compensate for the reduced airflow, the engine management system will use more petrol to keep the engine running smoothly.

Replace your air filter once a year to guarantee better fuel mileage.