I mean, I want to give you the typical advice and say be an active, outgoing member who has a long history of making quality posts and obeying the rules and setting a good example for other members. That's the answer I'd like to give and the one that I've seen given in the past. And certainly that has something to do with it. But being that kind of member doesn't guarantee you'll become staff by any means and plenty of the current and former staff have had a long history of serious conflicts with the forum and have been banned and have caused trouble lol. So I really just don't know.
Being a high quality member certainly helps your chances though.
Site staff is fundamentally different from forum staff. Typically one may apply to become site staff for whatever position they think they might be helpful. Forum staff is usually recruited on a need basis by other forum staff. It is a bit more rare. Unless you are meaning Event Staff or Competition Staff where those are made when there is an opening available.