Buzmania in the comments of Hypermiling ... it's more mental than physical:
Getting a car that has a consumption meter built into the electronics is a great idea. I drive a Volvo that has digital readouts not only for average mpg, but also shows you how many mpg you are getting as you drive. This is great feedback for driving habits. You can instantly see the effects of slowing and easing on the gas and coasting down hills, etc.
There are devices that can be added to your car that can give you this kind of feedback as aftermarket devices as well.
Simply changing your driving habits by driving slower when you can and planning your trips with a GPS or internet tools can save quite a lot of money.
These are excellent suggestions ... I know that Garmin GPS units have a trip computer on them that's easy to read and can help you figure out your miles per gallon very easily.
This will certainly be an advantage of the iPhone 3G with GPS .... maybe someone will make an application for the GPS that is more car specific - meaning that your car's average weight along with your personal weight + gas tank capacity could be filtered into an equation and give the same kind of data.
I also forgot to note the obvious in my Hypermiling guide - it's an easy way to be green. The less gas you use - the less gas you use. ;-)