How do you clean old nail polish bottles?

Fill them with nail polish remover, by pulling the brush partway out and pouring the remover down the brush post, or by using a tiny funnel. Let them sit an hour or so, swish the brush around to loosen old polish, then pour the remover out. Repeat as necessary until they are clean. If you are having difficulty getting the caps off (really old bottles!), soak the unopened bottle in remover until the cap can be turned.