Staff selects generally users who have fought Spam and Vandalism all across Wikia. You can see some requirements at this page. Now I'm quoting Josephyr :P :
This page contains good advice, but is kinda generic. The best criteria I can list are:
Have admin rights on a medium-to-large-sized wiki, proving that you are indeed competent to hold a position with rights.
Have experience dealing with trolls, spammers, vandalizers, etc... this can be done on your own wiki (if it's attacked enough), or through the CVN channels.
Be around Wikia for at least a year.
Not have any legitimate block history.
No (devious) alternate accounts.
Have a good relationship with other users. Don't be a jerk, or rude.