For me, it's been about posting regularly, and being focused on what I post. A number of people will see my channel and think it's not very focused, but keeping myself to my main themes has been good for catching viewers who want to stick with me.
Also, promoting on Twitter and Facebook and Instagram has helped.
The main thing, however, has been community-building. I make sure to visit the channels of the people I am subscribed to and paying attention to what they've posted, and commenting on their content to keep interactions going. After a while, it's not an obligation, because you can really forge some terrific friendships through the platform, and I'm happy to say that I like my subscribers as much as want them to stick with my channel.