At some contra dances the sets start to take on characteristics. There might be a set all the experienced dancers want to be in, or a set with the younger dancers. Divisions like this are probably not good for a dance community (though I'm not totally sure) but there are some times when they're just too much.

The key question is why are people are isolating themselves in certain sets instead of dancing in the whole hall? I think the main reason is usually wanting to dance with friends as opposed to trying to avoid people. Which means anything you try to fix this needs to let people keep dancing with their friends.

Which shows why this is pretty hard for individual dancers to fix. For example, if you who usually dance in one set you decide not to dance there anymore, there's a good chance you'll spend the evening at least partly sad about all the people you like you're not getting to dance with. And if it's a dance where people mostly book ahead while dancing, you won't even be able to get a partner from the other set to come over and join you. Similarly, if the caller tries to beg from the mic for the people at the end of a crowded set to join a shorter one, that caller is saying "you and your hands four should give up dancing with your friends to make for a bit less crowding". Not a very popular request.

So what can you do as a caller? Instead of trying to move a few people, move a lot of people. Walk down on the floor, go to the people halfway down the crowded line, and get them to start a new line next to the first. They're still going to get to dance with lots of friends, but now the lines are less crowded and more people can join in for more mixing.

When you have two sets worth of dances you can improve this a bit more by getting people to "rumble" at the top of the set. Break the popular line into two lines then dissolve the other set and get them to join on the end of the two popular ones. Tell people that when they're out at the top, they should switch lines with the other couple that's out. Now you have what's functionally one big line with everyone in the hall. Rumbling is also possible with more sets, but I haven't seen it work as well.

(Another option which might work is to disrupt the identity of the sets. After people line up for each dance, permute the lines: move the leftmost line to the far right. I haven't tried this, but I think it might be interesting.)