Tell them. In my experience, the friends that I really care about, and who care about me have all been incredibly supportive. Some of them - hell, most of them - find it hard to get their heads round the idea that I really am transsexual - I don't come across as effeminate in my male presentation - but once they get that this is something that really matters to me and that has been hurting me for years, they just want me to be happy and they all say they'll be there for me. (Easy to say that, of course: harder to live up to those words).
My parents, sisters and mother-in-law have also been really cool.
It's just my wife and children who've really had a problem. And as painful as that is, I can hardly deny them the right to be hurt. I am their husband/father, after all. They feel as if they're going to lose me, and they can't help but feel as though they've been cheated or lied to all these years because I'm not the man they thought I was. Only time will change that, if it changes at all.
But friends, well, it's a case of that old saying: the ones who mind don't matter, and the ones who matter don't mind.
