A sponge wet with water before the paint dries. Always have one handy when you paint with latex.

After the paint dries, use goo gone or any sort of solvent remover. Ask for a product your local home improvement store carries. You can also wet the area with water, wait a bit to let it soak into the paint, and try to scrape it off gently with a plastic scraper.

You want to be careful with any type of solvent. If it doesn't remove the finish - it could dull it.

Use alcohol and a nappy cloth - it will work

Mr. Paint