I reiterate what Ferret Herder said: do NOT give her any false hope. That’s just evil, because, if she’s got it bad for you, she’s not going to heal; she’s going to hope things’ll go back to normal, thus depriving her of a healthy future.
Otherwise, don’t do it right after sex, don’t do it right after she’s just done a somewhat permanent form of birth control (like Depo. Like my ex did to me. That was seventy five dollars I coulda saved), and don’t do it in front of a large amount of people. Don’t do it by phone, don’t do it by note, and don’t don’t DON’T do it via internet (unless you’re in a 'net relationship).
Also, the line, “It’s not me, it’s you,” should never be uttered, unless the other party has done something truly evil. Be the bigger man; say it’s you. It’ll save her a little bit of pain.