1. Respond to comments and messages. Keep your active subscribers happy. They're going to keep coming back.
2. Share on Facebook. TIP: Post your actual video on Facebook since Facebook is more keen to give it exposure if it's a video on its site rather than an embedded video. You can always leave a CTA at the end for people to come watch your stuff on YouTube.
3. Post on Twitter. Pretty straightforward.
4. Post on Instagram. I post the thumbnail of the video on Instagram and direct followers back to my channel via profile link.
5. Be active on OTHER channels. Post funny and witty comments. Get yourself some exposure on larger channels. Make sure you aren't directly promoting. Talk about the video you're commenting on. If your comment gets upvoted, then more and more eyes will see your comments, increasing chances of people clicking your name and visiting your channel. Yes, you can self-promote via commenting, but at least contribute to the video at hand. Don't demand it and definitely don't just leave an irrelevant self-promoting comment. It'll get you nowhere.