1. Decide to move on. You expend more energy holding a grudge than you do letting one go. Make the choice to clear up the mental space occupied by this resentment. Once you choose to move on, the grudge will gradually dissipate.
4. Forgive. Forgiving someone doesn't exempting them from their actions. It doesn't change the facts. If you have been legitimately wronged, then forgiving doesn't mean you have to forget. It does mean that you acknowledge that the person is human and that we all make mistakes.
5. Shift your focus. Look at the good things about the person. Find the positive in the situation. Maybe you learned a lesson and discovered something new about yourself. Changing your viewpoint will help you release resentment.
6. Don't feed the monster. Once you've voiced your resentment and committed to moving on, don't continually talk about the offense. If you find yourself thinking about it, mentally change the subject. If someone brings it up, explain that it's in the past and you don't want to dwell on it.