Hey there is a guide for this called: Five Helpful Tips on Growing a Successful Tribe
I also recommend you read the 3 guides that are linked on the bottom of that article.
Thanks for the info!
Right now, I think the big thing is getting the word out there.
Contact similar tribes, and ask them to link to you, and state you will do the same for them (see /t/linuxadmin rules for an example)
Post good content in the tribe, offer help where applicable, and just all around be cool. Also, get mods you can trust. Check their rep and past posts before offering them a spot.
When I get asked this I just say,
Post real, quality, informative content on a regular basis and every once in a while go out of your way to let people know your tribe is a thing that exists.
It also helps to run a tribe about something a lot of people know/use/need/find interesting.