+1 on ny_biker's suggestion. Check this site: http://www.cxnation.com/clinics/

Often, teams hold open practices that they allow others to join. That was the case in the Chicago area (my former neighborhood) and, to some extent, in DC. You could contact some of the local teams or race organizers. Clinics tend to happen at the start of the cyclocross season, though (August, September).

There is a great amount of videos in youtube. I like the how-to's from GCN:
https://www.youtube.com/playlist?lis...n6pX7L9lEa5Cwp