Spots are allocated based on the USF18 rankings:
http://www.naf18.com/joomla/index.php?option=com_wrapper&view=wrapper&Itemid=115

It's important to note that the rankings are based on results at regattas around the country. To ensure that you have a ranking, make sure your area rep is reporting results to Alex Van Brunt so he can keep the list updated. Best way to make sure you have a good ranking is to sail lots of events, and preferably bigger ones.