I believe SEO is most important after having interesting content.
SEO is search engine optimization. It's how you get your vid in front of people who may be interested in your content. Use the youtube keyword tool. You need to have good keywords in your title, description and tags section. Have links to your vids all over the internet. Forums, blogs, reddit, facebook, etc... Each vid you put out have a full description with links to your other vids.
I'm rather new. My channel is averaging 1,000 views per day and about 4 new subs per day. I got off off the ground by using places such as reddit and having good keywords that got my videos into the related videos of some big youtubers.
I'm still figuring out how to convert my viewers into subs. I definitely put an annotation at the end of every video with a link to a subscription page!