I don't know how much by, but the easiest, cheapest way to improve mileage and usually the first many people here will tell you is to inflate your tires. Most of us inflate our tires at or near the tire manufacturer's recommended maximum PSI (some guys even go over!). This will give you a bigger diameter tire which equates to more miles covered per rotation, and the tire will cause less drag as it rolls over the road (we call this lower rolling resistance, or LRR).
After that, the cheapest but maybe not easiest is changing your driving habits. Check out the link:
100+ Hypermiling / ecodriving tips & tactics for better mpg - EcoModder.com
Start with those and explore the site and you'll find yourself getting that 40mpg highway in no time!