There are a multitude of ways in which a decrease in a few MPG could be explained. The ones mentioned above are great starting points. I've found fuel octane rating to have a significant impact on MPG. Have you switched octane rating, or filled up at a new gas station? Most people would agree there is a difference in winter and summer fuel with relation to MPG. Possibly there was a switch over in your area.
A safe increase in tire pressure should increase gas mileage slightly. Less rolling resistance would equal less fuel input.
Gas Mileage Tips - Keeping Your Vehicle in Shape