I hear it said all the time, but consistency is definitely key... There is a good bit of luck to it too...
Personally, I make sure all of my content is tagged appropriately on YT. I also share on my Facebook page, and in Facebook groups based around my video content. I've had the best luck also sharing on forums, and sub-reddits around my content-type... Not spamming of course, but answering questions, posting topics, etc... To the luck part, something clicked a few weeks ago, and I went from 140 to now 461 subs... And it seems to be steadily growing each time a post a new vid. The boost from one of my reddit posts has created a momentum of traffic...
I'll also say that you need to differentiate yourself as much as possible. After much trial and error, I seem to have found a niche that I'm passionate about. I haven't seen much (if any other youtubers) doing the specific same content, and based on my channel growth in the last 2 weeks, there's definitely a market for it.