This may seem obvious, but most theaters miss it. There needs to be a calendar or list, prominently featured and easy to find, that shows when performances are and when tickets will go on sale for those performances. Many of my favorite theaters make this information impossible to find on social media, and a lot of their websites are also pretty difficult to navigate. On Facebook, create an "event" for each show so that people can sign up for updates about it. On Instagram or Twitter, post a link to the calendar so that people can look up what is playing and when. I'd be delighted if it were easy for me to find out what shows are happening when in my city, and when to buy tickets for them.