Okay this question is easy, as I've lead official clans 3 times now I can base my answer off those experiences.
For thread posting:
Have members post paragraphs or more only, one to five word posts are not recommended
No "Hey this is the 100th post" type things.
Have meaningful conversation, all your posts should not be welcoming some member to a clan.
For gaining members:
Mass recruiting can work, however don't fill your clan with 50 members in 3 days and have everyone organized.
Popularity is key, if you have popular members with a good profile behind them that helps a lot. I hate to say it but its not often that a bunch of nobodies that nobody knows get a clan to become official, popularity is key.
For ingame:
Host events, have hangout rooms that go for hours, just make sure your members are active ingame and hope the clan mods take notice.
This is a very short answer to your question and you can start by doing that, eventually I might make a good well thought out tutorial for it, but if you have any questions feel free to pm me.
Hope this helps