Acceleration - probably the biggest impact on fuel economy is acceleration, studies have shown that in urban driving accelerating slower and changing up earlier (as opposed to redlining it in every gear) can improve consumption by up to 40%.
Speed - next biggest fuel user is speed (where drag, and therefore power requirement is proportional to the square of the speed - which is why you can do 100mph with 80 bhp, but will need 400bhp to do 200mph). Above 30mph wind resistance really begins to become the major source of drag (obviously this will have more affect on a LandDrover Defender with the aerodynamics of a housebrick than a sports car with a Cd of 0.2). So all things being equal, the slower you drive the less fuel you will use.
Aircon- whether to use aircon is another choice, on an aerodynamic car like the Jazz driving with the windows open above 30mph causes a big increase in drag (unlike on the LandDrover where it has little affect) and it is probably better to open the windows around town but use aircon on main roads.
Weather conditions - the air temperature has quite an effect on fuel used, the colder the air the thicker it is (more drag), the engine also takes longer to warm up on a cold day, and a cold engine needs a richer fuel/air mixture to run - this can drag fuel consumption down to 20-25mpg for first couple of miles.