Though it is a difficult thing to forget people, especially when they are dear to our hearts; yet it is not impossible. Just like it is in breaking a bad habit, you replace with another habit, so when you wish to forget someone, simply get another person to take his/her place. This is so effective as it'll assist in forgetting previous experiences. Trying to forget someone means the person is either dead or is no longer in good terms with us, so replacing them with another is simply the best option. Who knows, the later could be better that the former.
"Out of sight is out of mind", this is true. In a situation where you're trying to forget someone: 1). do something that helps you get closure. Closure helps one move on. 2). get rid of reminders like phone number, photos, texts, mails, and so on. 3). find a replacement for that person. 4). keep yourself occupied always, you'd hardly have time to think of anyone.