It would be good if those with the highest MPG (especially 40+) to let us know how they achieved it.
I am at about the 95th percentile, currently averaged 37.29 MPG, having driven 17706 miles on 474.88 gallons of gas since I bought the car used in April 2007. I am hoping to improve this average. Most of my miles are commuting about 17 miles each way to work, suburban/country, somewhat hilly.
Besides the good handling, I chose the Focus because of the power train with good low end torque (down to the 750 RPM idle speed), and the gearing (50 MPH at 2000 RPM in 5th). Because of this low end torque, it is feasible to drive with the low RPMs, which saves gas. I generally up shift below 2000 RPM, often a lot lower.
I added a manifold vacuum gauge to give a readout of the throttle position relative to the engine speed, which I guess is like the engine load reading on the ScanGauge. This is about the 6th car on which I have installed this gauge. (They can be bought for about $20, much less than the $160 for the ScanGauge, but not as loaded with features.) I think the car gets better mileage using a moderate throttle (10"Hg), and it is best not to open the throttle too far (less than 2"Hg).
I put about 40 PSI in the tires to reduce rolling resistance.
I try to coast as much as I can, in neutral, engine running, down hills and to stops, or to avoid stops. (Drat those lights and school buses.)
I drive mostly with the windows closed for less drag and more quiet. I use the air conditioning, but try to switch it off while accelerating or going up hill (except when my wife is in the car). I keep the fresh air off (light on next to AC button) while using AC, so it doesn't pull in the humid air.
Some of these tips apply much better to manual transmissions than to automatic, since you have more control, and can smoothly shift in and out of neutral.