Might sound obvious but going easy on the accelerator is the main one.
The number of people I see that speed around in urban areas and then have to slam on the brakes 5secs later, is amazing. If they were to take it a little easier and avoid racing, especially when they are blocked up ahead, they would easily gain a few mpg out of their car.
Having a heavy right foot is a bad thing for fuel consumption.
I also remember watching a TV piece about a guy who advocates trying to read the road ahead for every situation. He was gaining major mpg inprovements. That is, as much as 10mpg extra just by watching his breaking, what gear he was in, revs, accelerating etc. Leads to boring enough driving i'd say though.