Magicians’ secrets really can’t be effectively copyrighted or patented, and trade secret law is of limited use, so how have professional magicians been able to keep most of their best tricks pretty much secret from the public? It’s by a set of strongly-enforced “common norms” among the profession, which allows “sharing” among those in the trade, but prohibits revealing more than trivial secrets. Violators are punished by expulsion from magicians’ organizations, and other techniques which make it difficult to continue working in the field.
From Jacob Loshin’s Secrets Revealed: How Magicians Protect Intellectual Property Without Law (pdf), via boingboing.