Prius Gas Mileage

[Jonathan Ryshpan]

The gas mileage on my Prius has slowly declined from about 40 mpg when I
bought it in 2005 (2d hand, it's a 2004 model year) to about 35 mpg
today.  Has anyone else noticed this?  

Any ideas why?  It could, of course, be just that I'm paying less
attention to driving for good gas mileage, or have let the pressure in
the tires go down, but I don't think so.

Thanks - jon