1, Keep it maintained
It might sound daft but spending a bit of money on your car could save you a lot in the long run. A properly maintained car will run far more efficiently than a neglected one. It doesn't all need to be done by your local garage either. For example, a simple weekly check of the tyre pressures to keep them properly inflated could save you lots in fuel costs. Fuel consumption can increase dramatically with under-inflated tyres - and they're dangerous too. Think about having some low rolling resistance tyres fitted next time yours need replacing.
Take off any unnecessary roof racks too; the drag they create plays havoc with your aims of saving fuel. As does any additional and unnecessary weight - so remove those golf clubs and other stuff from the boot.
