Be honest. That's rule number one. Have your reasons, exlain them clearly, and if you're being straight with her, she'll at least understand that.

You don't have to show any emotion to make her feel better. You don't have to pretend to be upset. In the past, I've broken up with girls and actually been quite cold with it - one in particular I told was getting too emotionally attached to me, and that I wanted to break up before it got really bad. She took it very badly, and still really hates me to this day. Fine, that's my lot.

At the end of the day, if I wanted to be Very Good Friends with her, I wouldn't have got with her in the way I did.