I've found this a great site for finding the resources necessary to enhance a portfolio. There are some really helpful people on the site that are able to give great advice and help to both experienced models and those just starting out.

I can highly recommend Adult Folio. Writing as a photographer it is a great place to arrange castings with a wide variety of models and also a good platform to show case your work. Very importantly the site is easy to use and any queries are swiftly dealt with.