What worked for me, strangely, was honestly being open to and desiring the inner resolution of old conflicts.
As soon as I invited that process into my life, I started getting weird reminders of this person out of the blue. Some song would come on, some friend would make a comment, and all those horrible memories welled back up, ruining my day. I hadn't thought of those things for years, either.
Being open to healing, I just let that happen. And as time went on, I had little revelations that made things easier. One of them for me was that I was more mad at a person who is still my friend than the person I thought I was mad at (the three of us used to be friends, and there was quite a bit of sabotage and romantic jealousy involved).
So now, unless I want to make myself miserable, I can think of this person without so much negativity, hatred, pity, etc. welling up in my system. I still harbor some animosity towards this person and the old wounds do still twinge, but if I ran into them some day, I don't think I'd have a crisis over it. They'll be just another old acquaintance I ran into on the street.
In fact, I used to stalk them on MySpace... and I haven't done that since.
LOL.