I have seen several posts on getting better mileage and I thought I would toss in my two cents:
You can waste your money on one of the 'programming chips' but most of them are just a simple $.02 resistor that fakes out the engine computer by changing the value of the incoming air temperature. If the ECU thinks it is 'hot' out it will lean the engine out and you get better mileage. That is until you burn a hole in a piston or other such stuff. Most vehicles today have knock sensors that retard the timing and if you run the engine lean it will retard the timing and there goes you gas savings.
Get a good air filter, get your tires pumped up to the rated pressure and then hypermile.
The best way to get better mileage is by do this:
How to Hypermile: 9 Steps (with Pictures) - wikiHow
I have gotten up to 15mpg with my Lex doing this.
I have seen posts about drafting off of a semi and I have gotten 2 to 3 MPG better mileage doing this. However, some semi drivers do not like you on their bumper. I have seen a truck driver wait until the very last second to switch lanes when there is a traffic slow down or construction. You then find yourself looking at the back bumper of a stationary car from 20 feet away while you are doing 60. Not good for all. It has not happened to me, thankfully. It is not a restful way of driving either as you need to be constantly alert.
I have often thought about a spring loaded magnet that flies out from your bumper and hooks unto the semi. You then reel out 20 feet of cable and just enjoy the ride.