First, an invite. I don't really look for an FC/guild to join unless I already know someone in the guild. So initiating an invite is a start, and I mean via tell/whisper as I don't usually read advertisements in chats and I turn off guild invites in any game that has that option (busy status in FFXIV doesn't count since it's not permanent).
Second, not be too exclusive. It's fine if you have a focus in RP or raiding or something else, but if you require in-character interview to join or you want to see parses or if you only want experienced raiders and not taking anyone new to raiding, then it's not for me.
Third, buffs, which are bare minimum even if FC is not active.
Fourth, being active together in some way is a bonus. I prefer to participate in PVE instanced activities, but I'd be happy just knowing the FC is active even if it's not an activity I would participate, like chocobo racing.