Like Chris says, the name of the game is reducing the losses in the system. Any time the throttle is closed, part of the engine's work goes into drawing a vacuum in the intake manifold instead of pushing the car down the road. We call this "pumping loss." Things like lean burn (stratified charge) and EGR force the throttle open more, reducing pumping losses. 1-2mpg is considered a "good" return for this on most applications. Don't look for a magic bullet here that will unlock 10mpg or anything.
If you're looking for fuel economy, you can also look at the trans calibration. Locking the convertor reduces some of the losses there as well. Making sure the convertor stays locked at cruise and most tip-ins will help. It can help to make the unlock points set lower than the downshift points at anything short of WOT, but drivability may be slightly reduced.