The Too-Many-Bands Tactic
My new favorite thing to do is to throw out more than enough bands. Say I have 22 kids and I need 5 teams. I'll throw out 5 bands of 5 different colors. This really throws off the kids who are good at math. But I let them know that some teams will have more people, which can sometimes mean less opportunity. And other times, teams will have fewer people, which can mean more individual opportunity. If it's really lopsided, I'll typically jump in and play with the team that needs a little extra help.
A great example of this is Ultimate Kickball. It works best with small teams. I usually do a rotation of four teams. Each team will go through a 2 minute rotation of Kick, Rest, Field, Rest. If teams have more players, they get fewer chances to kick and field, but they get to be with who they want to be with. On the flip side, the teams with fewer people get tons of reps kicking and fielding. And each player can choose how they prefer to work.