This thread covers what boundaries various forum members have: http://www.polyamory.com/forum/showthread.php?t=17813
As for how to set them, "I can't handle x so you can't do x" seems very one-sided to me. I would approach it more from a place of sharing and negotiation, as in "I've discovered that x affects me very negatively. Can we talk about what we can do together to help me with this?"
Maybe the answer will be a boundary, maybe it will be a slowing down or stepping back or a pause, or maybe it can be resolved by your partner giving you different things around communication and reassurance and attention.
I think that when you're talking about setting an after-the-fact boundary, especially one that may affect your partner emotionally or affect his or her other relationship(s), it's especially important for part of the equation to be you figuring out why you feel the way you do about it and if it's something you can work on.