This is a very good question. I agree with Amanda69 about limits being fluid (sorry I didn't quote it), for me depending on the day or situation my opinion or feeling about doing something may be vastly different.
And i really don't have anything better to add than maybe before play gets started, instead of having everyone naked and then say, "Ok guys...the rules for tonight's playtime are...."
Reading this makes me need to go look through the archives before I start a new thread...thanks for getting the wheels turning Prettylady!