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 & 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!
To make the cost of running the car less painful get a credit card that gives rewards, ie Natwest give points towards flights. Use the card for petrol only and always pay it off at the end of the month. The rewards soon mount up and then you can treat yourself
Have to second the diesel idea - having recently swapped from petrol to diesel it's amazing how less frequently you need to fill up, especially on long journeys.
Also Sainsbury's do money-off vouchers when you spend a certain amount on groceries (can be up to 5p off a litre and more I think). They don't do this all the time but it's worth checking your local one every now and then.
Also check out petrolprices.com who can use a postcode search to find your cheapest nearest petrol station. It's free to sign up.
I agree with all of the below, Morrisons etc. do 5p off petrol etc. Also try and go Eco..if you are going to drive somewhere what isnt really that far away, maybe try cycling? Walking? sharing?
Watch your speed - the "best" speed is 55mph, and as you raise to 70mph, you use 10%+ more petrol... crazy eh!! On motorways at 60mph roads I sat at 60mph. I dont carry extra weight (roof racks etc), and my tyres are pumped up well. Air con drains fuel so don't use that. I'm on petrolprices.com and the prices around here are always changing so it's a really good site, I get email updates from them as well. And, lastly, only use the car when you have to x
I've just read the other tips left here: there is 1 saying dont gear change as often ie. when accelarating. I've been told many a time this can damage your engine because if you from 2nd to 4th, although your car doesnt stall it doesnt actually have enough power (you might feel/hear it struggling) and this causes your engine more wear and tear over time. You can only skip gears when slowing down to a junction etc.