What is the origin of the idiom "a foot in the door" and when to use it?

The idiom "a foot in the door" comes from sales tactics, where a salesperson would use a foot to prevent a door from closing in order to gain entry to a potential customer's home or business. It has been in use since at least the 20th century. This idiom is used to describe a situation where someone gains a small opportunity or initial success that can lead to further opportunities or success in the future.