Typical generic MPG enhancers include but not limited to...

Make sure tires inflated properly..
Don't drive so fast
Don't drive around with alot of extra weight in the car.
Switch to synthetic oil
Ensure the car has a tune up , good air / oil filters
No dragging brakes
fresh fluid in the tranny.
Make sure your exhaust is not clogged.

Buy gas in the morning when it is cooler out , gas expands as it warms
so you are getting less for your money when it warms up during the
afternoon.

plan your trips

If the check engine light is on go get it fixed.

A car that is well maintained gets better mpg then one not taken care
of.

Efficiency is partly related to getting the proper amount of  air and
gas easily into the engine and how quickly the end result can escape
thru the exhaust pipes .
Thats why motorheads go nuts over high flow exhausts and air intake
systems

The rest is based on body aerodynamics, drivetrain design and upkeep,
and the idiot behind the wheel.

Also don't expect miracles. Don't believe bold face lies for MPG add
ons.