slack-etiquette
1. Use @here and @channel sparingly. Avoid them completely in large rooms like #general.
Does your message really need to notify everyone immediately? Probably not, unless it's an emergency.
Otherwise, maybe an email is more appropriate.
2. Invite people to a channel with /invite @user, not @ mentioning them in chat.
Mentioning someone adds to the chat log and unnecessarily adds an unread message or notifies members of that activity.
Using /invite won't notify everyone, which is a good thing.
3. Don't say "Hey".
After typing "hey", keep typing your message instead of just pressing enter. Otherwise, you prematurely distracted the person on the other until and they have to sit and wait as you formulate your next message.
4. Use email for topics that warrant long-form discussion.
Slack is great for quick replies, however it can easily be abused. There's something to be said for long, well thought-out prose especially when thinking about complex topics. The sheer process of drafting an email in a structured format may answer your own question without having to loop anyone else in.