I'm going to say that we got subscribers by just producing content consistently. It wasn't fast, but it worked. I would say it's better to see if people will cross promote with you (There are different threads for that).
I'd say focus on your content and what you are making. Try to make the best quality product you can. If there is a set story, then consider writing a basic outline to help you stay on track. Maybe even consider taking a week or two to create a backlog of updates in case life gets busy.
As you get more subs, you can do little contest things like "I'll do a raffle for a free sketch when we get to 15 subs" or something like that.