Instructables has an article on how to become a hypermiler. Reading through the list, I'm struck by how many of the suggestions are fundamentally dangerous. Among the things that they list are over-inflating tires, coasting with the engine off, wild swings in speed, removing emergency equipment, blocking the radiator, installing flimsy cardboard over your tires. Wow.

Also of note, that article recommends coasting by putting the car in neutral or turning the engine off. Yet with modern fuel injection, idling an engine while coasting in neutral can actually burn more fuel than leaving it in gear, so I would take the rest of their suggestions with a grain of salt.