Add a MPG Gauge to your car. This will allow you to instantaly see how different driving styles effect your fuel economy. If your car already has one then look at it and start see what happens when you back off a few mph on the freeway.

Nissan has recently announced it's new eco pedal which will provid feed back when you accerate. You can read more about it at Nissan's website.

Drive Slower

Slow down Sooner

Accelorate briskly while short shifting so to minimize the amount of time your accelerating.

Don't Tail Gate

Know where your going and plan a shorter route with less traffic or stop lights if possiable

Reduce Ideling time.  Some simple solutions to this are to walk in to the fast food joint instead of using the drive through. Turn off your car when waiting for someone to come out of a building. 

400hp and 30+ MPG

Further Reading:

Ecomodder 104+ Eco driving tips