Mileage on a 2001 Civic EX

Hello,

I've got a 2001 Civic EX. About 80K miles on it. Over the last couple
of years mileage has dropped from the 33-38 range for city/highway to
about 28/31. I've changed oil several times over the last year, changed
air and fuel filters, and kept tires inflated properly. What should I
look for next?

The book says plugs are not needed until 100K, but I suppose plugs and
wires are cheap enough to try. I haven't looked into EGR or PCV valves-
not sure what this car has in that department.

-Les