An End

How do you go back to just being friends? I started a relationship with someone I had been friends with for a while, and now I realize that I just like her as a friend. It's still a fairly new relationship, but I really have no experience in ending things amicably. Any suggestions on how to talk to her? (Obviously it'd be in person.)

I had thought about buying her a copy of He's just not that into you, the book the movie is based on, and then highlighting the appropriate chapter, but I'm really not the kind of guy who'd do something like that.