Realize that your subconscious can't really tell the difference if you're lying to it or not, so you basically just have to fake it to yourself and really "believe" that you will have a lucid dream or whatever you're doing. This is even used on children! Did your parents ever tell you to just repeat to yourself that you can do it when you were learning to ride a bike or something? Simply put, it is supposed to give you confidence, which actually helps a lot when it comes to things like lucid dreaming.