1. Make sure that your vehicle is serviced regularly. Things like worn spark plugs, sticky brakes, low coolant levels, dirty oil, and dirty fuel/air filters all add up to engine inefficiency, which leads to increased fuel consumption.
2. Check your vehicle’s wheel alignment. Bad wheel alignment causes more friction, which takes more power to overcome and results in higher fuel consumption.
3. Check for under-inflated tyres as these also increase resistance.
4. Close your car's windows when driving as open windows cause drag.
5. Reduce your car's weight by removing unnecessary items from it (think roof racks). Also, if you mostly do urban driving, consider driving with only half a tank of fuel.