There are many different model releases that are designed to accomplish different things, but all are designed to grant someone's likeness (or voice or both) being used for certain specified and/or unspecified uses. Some model releases are nearly as lengthy as Tolstoy's War and Peace. Some are very brief and to the point.
A google search for "model release" will probly yield hundreds of sample releases. Pick one that seems right for you, do some rewording and Voila! you got yourself a model release. Also, make sure--when you have the model sign--to get a copy of his or her I.D.