From what I learnt. It's time. And that theres no REAL way of speeding it up.
What you could be is spamming the shit out of social media and the forums whenever you can and constantly keep promoting your comic that way. Works for me so far.
I started around end of August last year and its Feb with nearly 30 odd subs.
Mind you I update weekly, With the only exceptions being the end of the year (I took a week off for Christmas and New Years).
So take that with what you will.