The clinics last from 1 to 8 days. They are advertised in Wind Surfing and other magazines. Local shops also sponsor occasional evening clinics on topics such as rigging and safety. There is nothing like a vacation to Maui or Aruba to improve your skills. Most windsurfing vacations have instructional packages.
Most localities have a local sailboard association.
You will get a newsletter, meet some great people, and help improve the sailing sites in your area. Surf the web. In 1976 at the North American Championships, sailors had heard that a 13 year old kid from Oahu could sail pretty well. However, this skinny kid blew everyone away in the free style by calmly flipping his board on its side, standing on the rail, and sailing away. The kid was Robby Naish, the place was Berkeley, California.