Call-to-actions and consistency. I think one of the best things you can do is just continue to upload at a steady pace, and like someone else already mentioned, use call-to-actions at the end to remind people to subscribe and comment and all that stuff. Sometimes, it's as easy as asking a question at the end of a video, a sort of "comment question" type of thing to encourage engagement and activity. I would recommend saving it toward the end, unless you can find a really creative and fun way to implement it in the middle (though I don't like this so much, makes it feel too desperate in my opinion). But if you do those you can hopefully ride the wave as long as possible!