Depending on the amount and where you live, you take them to small claims court. You will probably get the judgment, but you still may not get the money for a variety of reasons. You may have to go to court again and try a garnishment.

You can send your customer a bill (certified mail) stating that if it is not paid by _____* you put in the date*, court action will proceed. Or you can send them to collection (unfortunately this way you will need to give a percentage to the agency - BUT IT IS BETTER THAN NOTHING!!!)
Stay strong and persevere. Success always takes hard work, but it is worth it :)