As someone who has been running a successful weekly milonga for 20 years, I have some firm ideas on this subject.
The first step is to establish a weekly milonga. You must commit to being there every single week, week after week. If for some reason you cannot be there, line up a substitute. It is permissible to close on major holidays, or to take the summer off, if you let people know way in advance.
You need a knowledgeable DJ, someone who knows how to assemble a tanda, who has a wide selection of music, who can "read" the dancers, and who is capable of adjusting a playlist on the fly. If you plan to DJ yourself, take the time to learn your music, put together workable Tandas, and be willing to listen to feedback from longtime dancers
Establish a welcoming atmosphere. When someone walks into your milonga, they should be made to feel like an honored guest, whether it is their first visit or their hundredth. They should be greeted at the door by name, or asked their name so they can be greeted. And remember that people come to a milonga to dance. I invite a few people every week to come for free, whose specific job is to make people feel welcome, and to make sure everyone dances at least one tanda. I also encourage my regulars to dance a tanda every night with someone new.
Include a beginner lesson before your milonga. Encourage experienced dancers to join in.
Regular classes by a good teacher are an absolute necessity if a community is to grow. If your community has good teachers, support them. If not, try to bring in guest teachers from a neighboring community on a regular basis.
Avoid drama (This is not always possible, since there are always people who enjoy drama). But don't feed it. Don't gossip about the dancers, don't say anything not nice about other promoters, be positive and upbeat about everyone in your community. When it comes to the larger Tango community, use a cooperative model rather than a competitive one. Encourage dancers to support all Tango opportunities in your community. Do your best to avoid scheduling conflicts, so that people are not forced to choose between events. Reach out to other promoters to work together to avoid conflicts, and to let them know when you are adding something special. Invite other teachers to attend your events at no charge.
Make sure you have a social media presence, and nurture it weekly. Post your events, but also post all other Tango classes, milongas, and practicas.
Every now and then, plan a "Tango Family" activity - a road trip to another city, a cookout, or a night out without tango. Keep track of birthdays and mark them - a shoutout on facebook, or Birthday Dance at the milonga.
Growing a Tango community is a frequently thankless job requiring time, passion, and committment. But if successful, the rewards are worth it.