Here are some tips for long roadtrips and highway driving for the specific type of car I drive:
1) try to keep RPMs at or below 2000 on the freeways (most effective).
2) When possible, avoid interstate roads with speed limits 70, 75, and 80mph.
3) going up long stretches of highway, go slow and try to keep RPMs under 2500.
4) coast down hill as fast as legally possible without acceleration.
5) keep tires inflated to proper road conditions: when wet or slippery use normal tire pressure, when dry and hot use higher tire pressure, I use 36psi for both (AWD).
6) Drive at night or early morning when semi trucks are out, and avoid the masses of people going 75 to 85mph on the interstates and freeways.
7) Ease into acceleration, and coast to stop. Like driving NASCAR, conserving fuel with optimal performance.
8) When climbing steep hills for long distances, try to maintain constant speed and RPM.
9) use paddle shifters to up-shift when accelerating RPMs get over 28K.
This works for me, but may not work for everyone or every type of car. And, the time it takes to "Get there" will take longer when going long distances. If you're willing to sacrifice time and drive slower on the freeways and interstates you can save on fuel. I get really good gas mileage, but my gas mileage comes from 90% freeway driving.