Give us a little more info...
1) Do you have to start/stop much on your trip?
If so, accelerate SLOWLY. If not, try to maintain the same speed (ideally high enough to engage the trans lock ~ 45-50 mph) but do NOT exceed 60 mph unless you just have to.
2) Do you use the cruise control?
If you're on hilly terrain you may get better mileage NOT using it. If you're on flat terrain, use it like you're addicted to it.
3) Is your trip relatively level, or are there lots of hills?
Obviously, you won't get mileage as good if you're driving up/down hills a lot.
4) Have you put new plugs/wires in it yet? (you're about due)
The stock plugs/wires are known to be pretty well done by about the 25K mark. Buy some decent plugs (no need to buy the very most expensive ones) and a good set of wires.
5) What kind/size of tires are you running?
Stock (or close to it) size tires with a non-aggressive tread will typically yield the best mileage.
6) What tire pressure are you running?
Keep it at 35 psi Minimum for best mileage.
7) Are you checking your tire pressure regularly?
Maybe you're losing pressure occasionally? Check it at least monthly.
When the weather is cool enough, run with the windows down and air off.
Luis
ps. I've eeked out as high as 24 mpg from mine for mostly highway mileage on a tankful of gas by following these suggestions above.