The main reason why people would sub to a channel is because they like watching the content and want to see more of it, so naturally the better your content, the more people will subscribe. Also a regular upload schedule will help to make people aware of your videos, and it will make them come back too!
For promoting, think about your channel as a brand, how would you promote your brand? Have stuff like: a Facebook page dedicated to your channel, have a Twitter dedicated to your channel, a blog or website helps a lot, forums can boost your traffic immensely if you put your video in the signature of your posts, etc.