Words to live by: Orson Scott Card (borrowed from another blog)

A Crime of Heart... Breaking up with her and yet still hanging around, giving her hope that you will get back together when in fact you are merely lonely and using her till you find somebody better.

Tell the truth. Do it kindly, but do it. "I don't see this turning into any kind of long-term relationship, and I'm not going to waste your time or mine, when we ought to be finding someone else."

You might preface it with something decent and polite: "You're attractive and admirable. You're exactly the kind of woman I want to want to marry. But I'm not actually interested in marrying you, for reasons I don't understand and won't try to explain."

After you realize it yourself, the sooner you say it the better. And then get out of her life. Don't hang out with her. Even if you think she's "over you," she's not. There you are, a constant reminder that you didn't want her.

Don't send her little presents. Don't call her up and chat. Don't ask her for favors. Because that's what a guy who's courting a woman would do, and you're not that guy. Stay broken up. Go away.