Once your child has graduated from college, don't assume your financial responsibilities are finished. Adult children return home to live for a variety of reasons. Use the situation to help reinforce basic financial concepts.
Set a time frame. Don't let your child move in for an open-ended time period. Financial goals should be set and followed so your child is working toward financial independence.
Charge rent. There are increased costs when your child returns home - additional food, phone bills, utilities, etc. Although you don't have to charge a market rental rate, you should charge something. If you're uncomfortable taking money from your child, put the rent money aside in a separate account and use it to help your child when he/she moves out.
Put your agreement in writing. While putting everything in writing may seem too businesslike, it gives you an opportunity to clearly spell out your expectations and the rules of the house.