For general mileage tips, a good place to start is hypermiler HQ: http://www.cleanmpg.com/ (there is also a nifty mileage log there that will calculate and trend graph your MPG if you enter in basic data):



Remember that the numbers on the vehicle window sticker are not from GM, but the "official" EPA ratings. These ratings changed for the 2008 model year, and went down for just about every vehicle on the road. Go to http://fueleconomy.gov/ to find the numbers for your car. Here's what you'd find on the EPA site if you compare my summer car to my Aveo 5:



For the "estimated new MPG", the EPA is simply using a fudge factor to compensate for the new 2008 testing standards, which take into account more aggressive driving, inclimate weather, and the use of more power accessories (especially A/C). The godfathers of CleanMPG.com would ask that you use the published (ie window sticker) ratings for the vehicle, so in the case of my 2007, it would be 27/37.

I'll tell you that it's tough to achieve really good efficiency in the winter climate, especially when you're talking about below 0 ambient temps and snow. I mean, most people are going to run any car for 5-10 to warm up in my part of the world, and you're going to burn gas and go 0 miles just doing that. The best performance is going to come in the summertime, and I would guess that these cars can crack 40 MPG if well maintained and you drive them without excess weight and avoid the Mario Andretti stuff.