I've always felt that if you're going to talk to your wife/girlfriend about fetishes, you need to do it in an intimate/romantic setting.

Just doing it on the couch in front of the TV as you're watching football just makes things awkward.

An atmosphere needs to be created to help let your guard down so you two can talk openly.

I also think that you should kind of build up/working your way up to talking about your fetish. What I mean is don't openly just say right off the bat "I have a fetish for long hair".

Just kind of start off by saying something like how you love and appreciate a woman's body more than guys typically do and there are certain parts you find attractive/desirable much more so than those typical guys.