One of my dark secrets is that I am a trained hynotist. I actually learned the technique while I was still at school but since the I have had some lessons and indeed used it in an... ahem... semi professional way.
Most of the so called feminisation hypnosis tapes are just a rip off. For one thing this is supposed to be about becomming yourself and not putting on some trance induced big act, and for another most hypnotists seem to have a very narrow definition of what it might mean to be feminine (almost as narrow as 1970's gender psychiatrists in fact

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However what hypnosis CAN do for you is to help you relax and become most truly and comfortably yourself. In short it can help you let out all the stuff that is genuinely already there but repressed and burried deep. The things that you had to UNLEARN when growing up because they got you teased and bullied when you did them. These can all be unlocked by hypnosis. It can also give you that elusive self confidence and self belief.
Please note Seras has it right. You don't want to force your mind into an unnatural state. So this isn't about that at all. Instead it is about opening up your mind to it's truest and most naturally comfortable way of being. To allow you to be, as my mother always said, "the best you that you can be!"
I have used self hypnosis along those lines to very good effect.
I would, in principle, be happy to make up a few hypnotic MP3's along those lines for anyone who would be interested. I am doing one as a trial for an "IRL friend" I originally met through the forum, (no names no pack drill as I wouldn't want to her to feel embarrassed about being my tester). So perhaps wait a few weeks and then we will be in a better position to see whether it has proved helpful to her or not.