Passwords are easily the most talked about infosec control. Perhaps the simplest concept the explain and surprisingly hard to implement well. Allow a user to keep any password, without restriction, and she will keep her username as the password. Add complexity requirements and she will write it down.
Infosec professionals take every possible measure to get users to keep their passwords confidential. They provide guidelines on creating good complex passwords. They use analogies - ‘ A password is just like a key! Would you share the key to your house?’. They enforce password rules by building them into the systems. Try as you might, it seems that people and passwords seem to have a healthy dislike for each other.