It is important to keep your leftover paint, it may be useful for editing your work  or to paint new surfaces. You can keep the paint in its original container.

You first need to thoroughly clean the lid and container groove. For latex paint, cover the container with plastic wrap  before closing the lid tightly.

For an alkyd paint, use a layer of wax paper directly over the paint to prevent the formation of a "film" on the surface. Close the lid tightly and place the container upside down.