I figured most responses would say to pretty much just tell them flat out, which honestly is an awesome approach if you use it to tell people who are open minded and generally aren't judgmental. For people who you feel a bit concerned about, I'd go with a fan-based approach, introduce them to the popularity of the community and maybe show them a couple PMV's that show how FiM differs from the initial thoughts they may have had about simply the title of "My Little Pony."
I for one haven't really told any of my friends, just been juggling my two lives between non-brony friends and a Brony meetup group I joined a couple months ago and so far it's worked out just fine