An inside joke is proof you’ve been somewhere with someone. Guard those.

Inside jokes are compressed history. Three words land because of an entire evening nobody else was present for, so the joke is really a receipt proving you were somewhere together and paying attention. That is exactly why they cannot be manufactured, and why explaining one kills it on the spot.

Guarding them mostly means protecting the conditions that produce them. Go do something slightly boring with your people, because the jokes come out of unstructured hours rather than big occasions. Use the old ones deliberately too, since a joke nobody repeats quietly expires.