Don't drive over 65 mph
Pump the tires up to 90% of the max pressure on the sidewall.
Fill the trans-axle with a good synthetic such as Mobil 1 or Redline.
Look at grill blocking.
Make sure the car is up to spec.
Alignment is right on specs
Fresh spark plugs and filters
Make sure you have no brake drag.
Brake drag is when the pads are just
touching the rotors or drums producing drag.
I have a 2000 Neon with an automatic. Pumping up the tires helped me the most. I put in a grill block today. I'll let you know how that helps. Neons have great potential. They are pretty aerodynamic and underrated by the epa for mpg's. I drive mostly highway and get 34mpg. If you have an auto, stick with the fluid that's in it. It is really good from the factory and you can only get it from a chrysler dealer. It also cost $6.95 a quart. I had to buy some recently. Not mpg related but remove the big rubber weatherstripping against the firewall under the hood. Dyno proven 1 hp gain. Just a tid bit for you.
2000 Plymouth Neon