Online community and keeping active is your best bet. From what I've gathered, about 80% of my subscribers have told me they found my comic through other reader's lists. Your comic bounces up to the top of a readers list when your comic updates, thus when a reader/creator jumps on another person's wall they'll browse through the first page or so of that persons reading list and find your comic.
You'll have to manually go around and talk to people around the site. Don't start off the conversation saying "Look at my comic", that can be rather annoying. Just start it off with something else.
What I usually do is just ask if the person wants to chat, and I get to learn more about them, see what comics they make, and even get a friendship started (networking!).
6 times out of 10 they'll click on your avatar and jump to your page to get to know who you are and see you make a comic that way.
If the opportunity to talk about your comic pops up in conversation then do so. Often times when I talk to some of my subscribers they don't realize I run 3 different webcomics instead of just 1, so I'm able to nudge them in checking out the other 2.
Also, become an active reader and commenter. People will notice this (hey, I saw you like 2 minutes ago on another comic! Who are you? clicks on avatar). Users like Hobbes, Bo, Chuckles, Code, The Big Bear, Amet, Whomever, and Abs to name a few I'd say are very much acknowledged by the community despite some of them not running a comic series simply because of their participation .
It's all rather time consuming, but it's the only brute force method I can recommend unless you get the lucky wave of readers mystically flooded into your comic. Besides through this method you get to really interact with the community and even get some close buds out of it which is rewarding in itself