Among other things, a light right foot, conservative shifting, good engine tune, a smaller car, a more aerodynamic car, a lighter car, less "looks" accessories, being thinner, having thinner passengers, carrying less rubbish, riding in someone else's car, taking the bus/jeep/MRT/LRT, riding bicycles, and walking.
Bottomline is, there's no such thing as a "gas saving device". The car you drive, its state of tune, and your driving style primarily determines how much it consumes fuel.
http://www.fuelsaving.info