Great Question. Was working on it just Yesterday. Short Answer: 1. Learn about your audience. 2. Make videos that solves their problems. 3. Run "appropriate" youtube ads, with "appropriate" targeting. By Appropriate I mean, it depends from case to case. So your question in group should be, I produce videos related to X , intended at audience Y. What kind of campaign should I run with which targeting, to gain most amount of subscibers per $ spent.