To get people into sets I often used to use a promenade mixer.

Get them up on the floor
teach them to promenade

'Promenade all over the floor,
find two more and circle four,
find four more and circle eight'

Then you can stop them and go on from there.

Another thing that might help women callers, is try to sing in a lower key than you normally use. A husky 'Ruby Murray' type voice is more relaxing for the dancers.