I think a lot of it has to do with getting to know people, network, and playing around with social media.
I recently started a Facebook page and I put a little bit of money into it, so Facebook will show people advertisements of my page which links to my Youtube channel, and I've gotten likes on my pages and subscribers on my channel. That's with just putting in a dollar a day to keep it running. It's pretty easy. Obviously, the more money you put in, the more people will see said advertisement and, potentially, more people will like/subscribe.
Another I learned recently is learning how to tag your videos properly. I know it's pretty easy to tag videos, but tagging them with the right words for the right videos is pretty important. I'm still figuring it out. But some of my videos in a game series have hundreds of views in a matter of days while others are just sitting at double digit numbers.
Networking is important. While I've done this with other means in the past, I just recently started doing this for Youtube, because I finally decided that I was going to be more serious about my channel this year and see how far along I can go with it. I'm older than most of the Youtubers here, so I have more money to spend on things and I can travel more freely while others have to go to school. Anyway, something I did recently was go to PAX South, to a gaming convention. I made business cards for my small channel, and I talked to a lot of developers there for games. I asked them if there were free game codes that I could get from them, so I can show my viewers new and upcoming games while supporting them at the same time. In turn, they'll probably end up advertising my video on their game page and I'll get more viewers that way as well.
A lot of it is long term. You have to kind of stick to it and keep using repeated process before all of it pays off. It's a little boring at times, but I think with hard word and effort, people can make it work.
Sorry for the long-winded response! Hope this helps.
