Aside from people just enjoying your content, I find that people get more exposure when they interact with other people's work, and more exposure obviously increases your chances of people subscribing when there's more views.
Like sometimes when I find a comic/novel I love, I'll sub and comment on it, then I'll see the creator sub to my comic the next day. But it's kind of awkward, because then you're stuck wondering if they just subbing because you did (and I'm stuck hoping that they don't think I subbed expecting a sub back).
Oh, and definitely interact in the forums too -- I've found a lot of great works on there and after subscribing to the ones I enjoy, I've had some people do the same with mine.
Edit: I'd also recommend interacting with lesser known comics/novels! I think those creators will be more willing to check out your work because they're in a similar position, so they understand what it's like to be ignored or passed up. And it's just nice to spread the love, y'know? A lot of awesome works go unseen for various reasons.