69 dart 340 all stock

In addition to what`s been said you can make sure your:
1) brakes don`t drag
2) wheel bearings are adjusted right
3) front end is aligned as well as the rear differential
4) tires are good quality radials (not wide) and inflated correctly
5) excess cargo is kept to a minimum
6) oil is changed frequently

As mentioned earlier a good electronic ignition, set of headers, and a spread bore carb, will all help mpg as well as a viscous fan, low set of rear diff. gears (like a 3.23:1 or lower) and a lock-up torque converter.
Lastly, buy yourself a vacuum gauge and mount it where you can see it. Drive the car around and maintain the most amount of vacuum you can.