It took me some time to convince my wife to cuckold me. Through out the years before, my wife would sometimes play along, and sometimes get angry with me for bringing it up. Then it happened. It was what she and I both agree, a special situation. It happened between my wife and our best friend/neighbor. It went on for about a year and then stopped. As with many here, the fantasy involved a black guy, me getting to watch...etc. When it came down to it, our neighbor is white, and I never got to watch. I took what I could get, and I suggest you do the same.
I obviously still have the fantasies, but at this point she does not play along at all. She gets upset or just ignores my comments most of the time. She tells me at this point its just not going to happen again.
My suggestions to you are:
Talk to her outside the bedroom. You two should be able to speak about it, if she is serious, without getting under each others skin.
Make sure you two set up ground rules.
Make sure she is not doing this just for you, its for both of you, well to honest, mainly for her to get the "main course" and you the "left overs".
Basically, you just need to have a "real" conversation about this. Not an angry, charged up conversation.
In my opinion, the best type of cuckold relationship is the "loving cuckold relationship". You might want to at some point show her this site.
Good luck, keep us posted.