1. Lovingkindness practice
This one’s pretty obvious — if you’re a meditator at least. You can simply call to mind the person you’re resentful of, and cultivate good will toward them. We have a whole section of this site devoted to teaching the metta bhavana (development of lovingkindness) practice, so I won’t say much about that here, except that it does work! When I first started practicing meditation I had a lot of problems with resentment, and I was often surprised by how quickly my anger and resentment toward someone would just vanish.