Got to
http://www.copyright.gov/forms/ for the forms. For still pictures, you'll need to print and fill out Form VA and read
http://www.copyright.gov/register/visual.html. Check the "deposit requirements" for "Pictorial or graphic works fixed only in <U>machine-readable form</U>". Burn a CD with your images on it and give the collected images on the CD a title (can simply be "Assorted Nudes, Volume I"). Make a backup CD, photostat the filled out Form VA and mail the originals with $30 certified mail (copyright registration begins the date the Form is received).
Videos, either on tape or disk, have to be handled seperately because they require a Form PA (don't use the short form!). Everything above applies PLUS- you need to do a cover sheet as well that states the names of the director, producer, cameraman (can all be one person, yourself, but you have to state this) and whether or not they were compensated for their efforts. This is to ensure their isn't a later cross-claim by someone else involved in the production, so keep it simple, say you produced and directed it and paid the cameramen hourly wage. Be certain there is no background music that you don't have the right to use!