Honestly, you're foot is the biggest saver! It took me a while to learn this, but if you can resist the urge to listen to that big brute under the hood sing your gas mileage will soar up.
When I first got the truck I was around 13-14 cause I LOVED hearing the rumble....but I sit closer to 16 now that I can drive it more conservatively. (my daily drive is 60% city, 40% hwy)
Muffler, CAI, and tuner will help but once again only if you can keep your foot out of it. I've done both muffler and high-flow filter and it really hasn't amounted to much other than teasing me with a meaner growl
So definitely try driving more conservatively, but don't be afraid to drop some mpg's when Ford's and Chevy's pull up at a light.