At our house, we modelled the bringing in of a new love after the major rules for bringing a new kitten into and already established and "claimed" cat household. (YES, you'll see many feline references in our family)
Most are the regular ones that we all assume...to reinforce our feelings for the established loved one etc. But one major one that I rarely see mentioned among Poly's is that the new person has also see when you are being affectionate with the established person. If there is an intention of being a "family" or something similar, the New Love must also relaize and visibly be aware of the established couple or group. This is not intended to leave anyone out, but it worked well for us, and neither my husband or new love felt neglected in the process. Instead it is a means of "clean, honest communication." The new love is aware of the established commitments, but until it's brought up in front if them, it's not a "reality."
I've always wondered why I read so much about folks choosing to focus on one partner or another in regard to an event or time period. I think it's important that each of them see me with the other person.. It tends to eliminate some of the "mild neuroses" that are natural human traits in relationships. (Well he or she is with ME now, not her.) But, then again, I prefer our family to not be the "taking turns type." I much prefer the big family hugs where we all get a chance to be in the middle rather than loving them one at a time.