When one asks their friends and family members to give tips on how to get your ex back most eventually suggest to remind your ex just how much you love them and how empty you are without them. While this may sound quite plausible do understand that your ex wishes to no longer be in a relationship with you and therefore trying to get them back into one will only result in them going farther away from you.
What you should do on the other hand is to try and get him or her to realize their mistake in leaving you. Try to teach them what it was to be with you and how they are missing out without actually telling all these things to them. Maybe then they might want to crave those three magical words from you. When the time is right you may go ahead and spill your guts to them but until the defences against you are down you cannot tell this to them.