Sometimes a "veteran" member of the group can say, just as they see that certain person taking a breath to sing song #4, "[name], we haven't heard from you in a while (or "yet") -- is there something you'd like to do?" This is made a little easier when there is a recognized leader, as Jon mentioned, but usually there is a kind person who can, as the jargon goes, facilitate.

Sometimes you just have to take the bull by the horns and in a private moment (*not* during the circle, but during the break or at some other time), say something to the "offender." If you put it in terms of how some people are shy or nervous and need extra time or encouragement before they "jump in", that doesn't point the finger directly but hopefully will get the message across. Of course, some will use a more subtle method: "For Pete's sake, shut up and let somebody else sing a song!" Depends on the personalities involved, of course, and on how big a problem it is.

I really like black walnut's arrangement of having a turn-taking circle before the break and unstructured time after. Sounds like the best of both worlds.

Aloha,
Mark