Aerodynamic resistance is proportional to the **square** of the velocity. At speeds under 35 or so, other factors come into play, but above 35 it is effectively all about wind resistance. Simply, as Hot_Georgia says, slow down as much as you can on the highway.

I strongly recommend you track down Wayne Brown's Palm program that simulates Prius II mpg under different driving conditions. If you do not own a palm pilot, OS simulators for windows and Macintosh exist. It is a *real* eye opener.