Software sales methods

I have written a point of sale (POS) software for restaurant internal
use to take customer orders, post food/drink orders to kitchen/bar
printers, calculate the bill when the customers leaves, etc.

I want to ask the promotion medias to let potential users to know the
product.

What I have known includes sending promotion letters by mail, by email
and by faxes.

I want to know whether anyone have other effective methods?