What you need to do is drive like the EPA's FTP75 drive cycle.
You need to have a lot of stop-and-go driving, from 0 to 30mph back to zero, with moderate acceleration. This will maximize your regen braking and recharge your battery.
And, you need to drive an average of around 55mph for the highway portion. All of this needs to be done on relatively flat ground, with minimal headwind.
Another trick, you need to limit your trip to the length of the EPA75 drive cycle, which is 1874s long (31.2 min). If you drive longer, you'll use more gasoline, and get lower fuel economy; if you can drive shorter, you'll see even better numbers.
I'm only partly joking. The biggest factor in fuel economy at highway speeds is, well, the highway speed. If you can slow it down from 70mph to 60mph, you'll see big improvements in your fuel economy.
(Graphic courtesy of Detroit News)