Wiki science/Vibrant User Base
Sometimes a link to a short page of "tips for new users" is included in the initial welcome. (Please don't require new users to read and understand hundreds of pages of arbitrary "This is How We Do Things Here" text before they are allowed to write anything.)
Encourage new users to be bold.
In many cultures, there is a tradition of "This Is How We Do Things" that stretches back so far that no one remembers the real reason they choose that particular way.
In wiki cultures, it's simple enough to document various decisions, leaving a place for further discussion, and making it easier to revisit that decision and perhaps choose to start doing things a different way (if a better way is discovered, or if situations change so that the original way is no longer the best).