How can I learn to let go of resentments?

I sometimes find myself holding onto feelings of resentment towards people who have humiliated or harmed me in other ways. In some of these cases, the people who harmed me have no idea that they were in the wrong and will likely never realize that, yet, I still sometimes feel intense bitterness about them, from time to time. How do I find healthy ways to properly manage these resentments?