I'm not sure on how the law works in the US, but I'm assuming it's fairly consistent with Dutch law. As far as I know, if you make a portrait for someone - that someone owns the distribution rights to that picture, thus limiting your rights as the author. How much you want to limit the client's rights is often arranged in a quitclaim. Usually clauses regarding the following are included: duration of contract, description of assignment, distribution (number of prints for example), exclusivity (are you allowed to sell your photo to other clients or use in a book?), processing (is the client allowed to change the picture, if so: how?) and finally what happens if something goes awry beyond your power (fire burns down your hard-drive). I'm sure there's a photographer's federation with a sample quitclaim somewhere.