There are lots of ways to achieve that, but the ones that are always told are the following:
-Regularly post content, to keep your subscribers (even if aren't much) entertained. This will prevent you from losing subscribers, can encourage your subscribers to share your videos around, and of course gives you more chances to be seen by new viewers.
-Interact with other Youtubers, if possible doing the same as you, and collaborate with them. Even if the video you create together is shared in their own channel, you will still get some exposure which could lead to getting more subscribers.
-Have a unique, eye-catching logo & banner, in order to attract people to check our your content whenever they see it.
-Comment in others' videos, specially from smaller channels, because those tend to subscribe to whoever likes their work. Although, it is also a good idea to comment in popular videos because those are seen by a lot of people and there are chances somebody stumbles upon a comment of yours and checks our your channel.
-Make sure you're using the right tags/keywords in your video, which lead to being found more easily whenever somebody searches those words. Aside from that, have a thumbnail that "pops", with text that makes people want to check out the video.
I think those are the basic techniques that I use myself - hope they can help you out
