One
Break up before you dislike your partner enough to make it easy.
Two
When breaking up, take all the responsibility and do it in as loving/compassionate way as possible—it sucks to be broken up with, to have spite and vitriol in addition is just insult to injury.
Three
Generally, if there is a break up, it should stay broken up.
And, in case three is a bit too harsh for you, a bonus on the topic of second chances:
Second chances depend entirely on how the first chance went, and are most likely to succeed when commandments one and two were followed.
