Depends on your personality a good bit.

1) Work. You can just work hard. Speak at Meetup Groups, WordCamp (or similar), Trade Shows, Niche Conferences. Speak to many people who are interested in your topic + plug your YouTube channel during your talks.

Or better plug your Website with an optin device + publish content on YouTube + many other venues.

2) Create controversy. This is niche independent. This works whether you're a militant Vegan, Trump supporter, Liberal troublemaker.

3) Run paid traffic directly to video. This depends on traffic source + their content guidelines, which seem to change daily.

Likely you'll have to embed your video on your site, wrapped in content + run traffic to your site page. Likely many people will click to play your video on YouTube.

4) Most importantly, give people a good reason to subscribe + tell them to subscribe + what benefit they'll receive by subscribing.