How to trade draft picks in a keeper league and prevent tanking?

Trying to open up trading in a 12 team keeper league. League has been active since '04 and we’re not allowed to trade draft picks. There is barely any trading and I think trading draft picks would open it up. We can keep up to two and they are slotted as first and second round picks. Trying to figure out ground rules to prevent tanking and stockpiling. Any suggestions?