It can only happen when you are in a relaxed state, without fear. It is a natural occurrence, but the mind will stop it unless you "become one" with your natural state. The best example I can give (and I know some people find this an odd analogy, but it is metaphorically accurate) is a male erection or a female's state of sexual arousal.
It happens on its own when the conditions are right -- and no amount of thinking or hoping will "force" it to happen and fears and tensions can keep it from happening.
This is a close parallel to the pre-conditions for "astral projection". You don't teach someone to "be more psychic" -- you help them remove all the clutter "between them" and their natural, intuitive state.