Drive sedately.
Of course the best way to save petrol is not to drive at all, although that's not always an option. But when I see someone get in their car and drive 50 yards only to park again it's hard to have much sympathy for the fuel they use. Unless you're disabled you can walk 50 yards or more. In my area I see people get in their car at one place, drive literally across the road to another place and then stop. Sometimes people give me funny looks when I walk All The Way across the road, rather than dealing with three stop signs in less than 100 yards.