I used to get just about 20mpg in the country roads in Kentucky, in a 4 door 2wd.
It had the (previously mentioned) exhaust work done, a larger MAF, the airbox was modified, and I kept that puppy tuned up (!).
Synthetic engine oil helped me too.
Lucas fuel injector cleaner about once a month seemed to help also. The little bottle is WAY too expensive, I bought the big bottle and refilled the little one I had every time it was ready.
I'm including a link to summit racing, and a dynomax exhaust "CAT back" system. You'll have to figure out the CAT problem I guess - the one listed might be for a four door, so check for the right one!
The kit is like $125, with a $75 rebate right now. (!)
http://www.summitracing.com/parts/WLK-17415