Gurus make naturalistic observations and share iformation. Sometimes they see some kind of literature and test a hypothesis. Most seem to be aware and follow the work of others, and there is a great amount of carryover in what is taught. This information is then internally shared, tested, etc and eventually becomes public. Additionally, millions of men in the world do the same and sometimes publish their findings or give info to other people who publish their ideas.
All of this is extremely subject to a huge number of biases. However, the beauty is a healthy dose of skeptisism is bound to be present in the masses. The gurus who give total crap advise will likely be eventually washed out. Take Ross Jefferies for exampple who now only comes up as the butt-end of jokes at this point.
Many of the ideas I got from Deagnelo, Mystery, Swinggcat, Shark, TD, etc. were very good. The beautiful thing is you can test their ideas for yourself, see if you're able to replicate positive results other's have seen, and make adjustments as needed. You are not forced to use anything.
