Streamlining your vehicle is one of the most elementary aspects of hypermiling. Hypermilers will go to great lengths to make their car more aerodynamic and reduce drag. Your car can easily have as much a couple hundred pounds of excess weight just sitting there. Try going through your car with a fine tooth comb and removing any extra junk you don't use on a regular basis.
Sandbags, or heavy bags of solar salt, winter accessories like windshield scrapers, books, extra bottles of motor oil and other car maintenance products, water jugs, unused child accessories like strollers, or even piles of papers and manuals in your glove compartment are just a few of the things you might find. What about garbage? Are you the type who throws bottles, wrappers and cigarette packs on your back seat floor? Give your car a thorough cleaning and you can seriously increase your gas mileage. You could even consider removing your spare tire and equipment. Although it's generally a good idea to keep a spare handy, if you aren't going on any long trips, and if you have a AAA membership or an equivilant in case of emergency, removing your spare tire can free up some extra weight. Are you driving a Van or Minivan with several rows of seats that you don't often use? Remove the extra seats and store them in your garage. Once you know how to easily remove and replace seats, you'll be able to do it in a snap, and removing those heavy rows of seats is a sure fire way to streamline your vehicle.
Although it's not going to double your mpg, smart hypermilers know that a streamlined car is one of the basic rules of hypermiling. There's no sense in bothering with any advanced techniques if you're still driving around with hundreds of extra pounds of weight in your vehicle. This is a real simple hypermiling technique that will help you get on your way to fuel efficiency.