Is your content interesting and engaging? Does it stand out in some way? If so, spread the word on your blog and website. I also recommend collecting emails and notifying your user base when new content is posted. Additionally, remind people to hit the subscribe button in the video itself. Once someone subscribes, they will automatically be notified when new content is posted. The more effective channels you use to push your YouTube channel, the more chances you have of increasing its visibility.
Let me give you an example - I am a fan of a YouTube channel called Active Self Protection (ASP). The founder is a U.S. Navy vet, John Correa. His channel now has a subscriber base of well over two million viewers. He did not achieve this level of success by accident. Check out his channel and see how he promotes it - on his website, his email list, the channel itself (literally in every single video), etc...
I am sure you are using popular keywords and making attractive thumbnails for your videos.
If you aren't, try to focus on that.
You can collaborate with other YouTubers or run ads to get new traffic to your videos and ultimately get subscribers.
Try to build relations with your social media audience and then share your videos with them this will surely help you to get long-term/loyal subscribers.
For increasing subscribers, the best thing is the content.
If your content is best or relevant then, all people subscribe to you & make you famous by sharing it with all people. Put efforts into making amazing content or the best video quality.