There's a couple of different ways this is typically structured, for instance:
) - This isn't very popular among the photography community since photographers don't like giving up the rights to their images. It's generally expensive to do on a single-time basis, $100/hr, $200/hr or more for a known photographer, unless it is a part-time or full-time position at a firm (with benefits, some job security, etc.)
shared rights - You both maintain rights to the images for your own purposes. These rights may or may not overlap, Ie. as a photographer you would maintain rights to do what you normally do with the image, while the model is granted rights of her own to do what she wants to do with the image. It's best to specify both -- for contract purposes. Remember that without a contract, normally the photographer gets all rights and the model doesn't have any, so you have to grant rights to the model as well as possibly forgo them yourself.
partnership or royalty - As the author of the work, you get a portion of the gross sales, or a portion of the gross profit made from the sale, or some other form of compensation.