I read a few interesting thoughts about handling the peril and/or ending of a (e.g. poly) relationship at: http://mind-crush.com/2013/11/lesson...g-sports-cars/
Re (from copperhead
"I am trying to find out what kind of difficulties people have in transitioning from mono to poly or adding new people and what are the best strategies to avoid anyone from getting hurt in the process."
Off the top of my head, good/plentiful communication (important in life in general) is especially important in polyamory, and certainly important when transitioning from mono to poly. The better you understand how to communicate well, the less hurt you'll be in danger of.
But I think it also has to be said that hurt is one of the risks you take when entering into (any relationship but especially) a poly arrangement. The rewards are greater but the stakes are higher. This is not to say, "Too bad, just deal," but it is to say that the hurt can only mollified, it can't be eliminated.
Again, practicing good communication (and getting better at it in the process) is your #1 strategy for reducing the amount of damage a "poly failure" will cause.
Now, communication is a two-way street and if one party refuses to try to communicate honestly and considerately, then the other party has to protect themselves. Usually this means putting as much space as possible (as soon as possible) between you and the "hardhearted party."
I guess it seems like the answers to how to deal with (the danger and occurrence of) poly failure should be clean and abundant. Unfortunately, in practice the answers are messy and sparse. Much also depends on the details of each unique situation, as no two breakups are alike.