Once a bike is in motion, it takes about 15 horsepower to keep it rolling. That's a very rough figure. Most of the power required is to overcome aerodynamic drag first, and rolling resistance second.
Probably the best way to get better mileage is to, as thetable suggests, switch to a smooth tire and then address aerodynamic issues. Saddle bags, top boxes, nerf bar bags, and being a physically large person all contribute greatly to drag.
There's not much we can do about our physical size, but running without the bags and boxes whenever possible will help.
I would suspect that those reporting mileage in the 50s and up aren't north of 6' tall and much over 200 pounds, run the bike pretty naked, and are easy on the throttle.
I'm a Clydesdale, run saddle bags, nerf bar bags, a top bag, and road it like I stole it. My mileage is averaging 43mpg. The best I've ever gotten was 52mpg, which was downhill with a tailwind.
For an easy way to track your mileage, check out www.fuelly.com