There isn't a standard system. The pins stay on the lanyard until they are traded or someone decides to "freshen" it up. For each area there is a place back stage where the pins are kept in and they refreshed from there. Sometimes it is just taking the mulitples off and putting new ones on and most of the time it isn't anything special. What gets put on often has a lot to do with who is doing it and how they feel about pin trading. Some will take the time to find pins that they think people will like and create a mixed assortment lanyard.