Pretty much what Rob just said.
A MC with a light touch does help, or there can be one or two either more experienced or over keen people who monopolise.
Also it's reassuring for novices like me to be able to ask to be passed over this round if you're feeling insecure (read 'havent drunk enough to overcome your inhibitions yet').
Whereas I love the organised sessions I attend, some of my favourites have been spontaneous sing-songs at re-enactment events after hours. The great thing about the organised sessions is that they can really bring your singing and playing standard up so that people enjoy what you do at the impromptu events!