I'm getting around 5 MPG less now than in summer. I attribute it to the cold weather and school traffic. I got as high as 41 MPG last summer and now I'm getting between 35 and 36.
Driving style is probably the biggest thing you can do to increase mileage. If you think about it, using your brakes wastes energy as is using the low gears to get the car back up to driving speed. So don't tailgate, look ahead and try to time the lights when you drive. Also fill up your tires. I try to keep mine around 38-39 lbs (warm).
2010 Cobalt XFE
2006 Chevy Colorado 4x4
2005 Toyota Corolla
2013 Kubota M5640 SUD