What are some of the slack etiquette standards you have in place and how do you best enforce those?

We have an internal facing wiki which has how to guides and a slack etiquette guide.

When we have big "set piece" activities that make heavy use of Slack people are reminded how best to use Slack, keeping messages in threads, which emojis mean what etc.

Also worth noting that Slack is a work tool so is covered by our standard terms of employment, same as email. This helps set expected standards of behaviour.