For me, getting the first hundred was the hardest. I basically begged everyone I knew on Facebook to subscribe. Even going so far as to personally message some of my Facebook friends. There were also a few sub for subs in my first 100 subscribers. I've actually seen many of those fall away as time has passed. Once you get that first 100 and claim your channel name, you start to feel like a "real" channel.
My fastest uptake of subscribers happened when I saw one of the large channels in my niche doing a live broadcast on YouTube. Because YouTube pushes live broadcasts so hard, they naturally get higher views and often wind up on the homepage. (That's a whole growth technique in and of itself!) I joined their live chat and just started asking intelligent questions that were on topic. The channel owners had seen one of my videos because I had reviewed one of their recipes on my channel, and they loved it. They gave several shout outs to me during that live feed, and within the next few days I had gained hundreds of subscribers. I have gone on to enteract with that other channel a fair bit, in their comments and in their live chats on Facebook, YouTube and Instagram, and this has led to a huge swarm of subscribers for me. And they all seem very active and complementary and engaged, and tell me that they love the content.
I have also spent about $100 each on Facebook and YouTube ads. Those definitely helped with view counts on videos as well as subscribers. Having a short, effective channel trailer that I could use to run ads targeting specific channels in my niche really helped me.
So I think a combination of good content, a few targeted AdWords campaigns, plus a strong social media presence outside of YouTube really helps. I'm still a very small, very new channel, but I've gained almost 1,500 subscribers in about three months. Good luck!
