Keep the car well maintained and the tyre pressures at the correct level. Don't rev too high when moving off and when stopping take your foot off the accelerator a bit sooner to slow down gradually. Don't carry too much weight in the car and remove bike or roof racks when not in use.
Shop around for fuel, compare prices if you can, or shop at supermarkets which offer money off fuel.
Check out www.petrolprices.com.
You can register yourelf on this site for petrol with your postcode and they will regularly update you with where the cheapest fuel in your area.
Try to avoid making really short journeys and wherever possible walk instead. I try to do my food shopping on the way home from work which saves getting the car out again.
It is a good idea to minimise your use of the air-con and heaters which use up power quickly. Having the windows open is meant to affect the aero dynamics of the car (or something technical) which could make the car run less efficiently.