Connect with like minded YouTuber's, attempt to collaborate with them if possible. Comment on YouTuber's videos, aim to get the highest rated comment for more visibility. Example : Gaskings puts out a video of a GTR Flamethrower. Comment something like "Where the GTR Fanboys at?" and you will see 1000likes in 24 hours. Join related forums and other social sites and cross post all of your content.

The most important thing is to engage with your audience, try and connect with them and show empathy. It's no good in having 50,000 subscribers and only 1000 of them come to watch your videos every time you post.

I've basically stopped doing all of the above due to time constraints in my life and have gone from 200 subscribers a month to less than 40 gained.

Hope this helps somewhat?