Lucas oil! 0w30 naturally! I get easily 20-30miles more on a full tank. About £30 for 5lt (enough for an oil change)
Also cars more than2 or 3 years old may benefit from Cataclean which is an additive that goes in with your petrol and usually returns improved fuel economy and throttle response (depending on how clogged up your catalytic converter is!)
Easily the biggest change of all however, is changing your driving style, and I don't nessecarily mean slowing down
Better use of gears and looking MUCH further ahead so you can plan your driving will help loads!
ie: if you're approaching a car ahead that is travelling slower than you, back off the accelerator and match their speed rather than maintaining your speed and using your brakes to scrub off the speed at the last minute as you get close up to them. Sounds obvious but things like this make a massive difference!!! My fuel economy improved loads after a few advanced driving lessons! The more you drive using acceleration sense and less brake pedal, the better!