Keep the car well maintained and the tyre pressures at the correct level.
Don't rev too high when moving off and instead of having to brake suddenly at junctions etc. when you know you have to slow or stop, just take your foot off the accelerator a bit sooner to slow down gradually, using your gears to slow you down too.
Remember that short journeys use more fuel.
Don't carry too much weight in the car and remove bike rack, roof rack when not in use.
Shop around for fuel - compare prices if you can or consider a credit card which offers money off fuel (eg Asda - sorry don't wish to advertise but if you use a lot of fuel 2p a litre off is very useful)
Check out www.petrolprices.com
You can register yourelf on this site for unleaded or diesel with your postcode and they will regularly update you with where the cheapest fuel is in your area. I usually buy my fuel in a different area from where I live because it's cheaper but it's not worth it unless your going there anyway!