
There are numerous ways to do this, and instead of rehashing what has been already written I'll point you over to this list of 105 hypermilling tips at ecomodder.com.
This list contains some known tips such as: Close the sunroof at higher speeds, avoid peak traffic, and combining errands into one trip.
There are also some lesser known tips like: Drafting a vehicle in a crosswind and avoiding idling upon startup.
Then there are some of the crazier tips: Listen to slower music, avoid parallel parking, and driving shoeless.
It's a pretty interesting read, some tips you may use, some you probably won't. Odds are though, you'll learn a bunch of new ways to get better mileage. Tips that might come in very useful once the pumps hit the $5/gal. mark, if not already at the $4/gal. mark.
Check out the 105 hypermilling tips to decrease your fuel consumption!