Self-promotion is a weird science that I don't think I have fully grasped yet, but here are the things I've noticed that work for me:
1.) Update people on what you're doing. Have you posted a new page? Let people know! I crop out a panel from the new page and use to to make the link to the page look more inviting on Twitter+Tumblr, and that works better than just posting the link.
2.) Don't be afraid to be excited about your comic on social media sites! Really like the page you're working on now? Tell people. Squeeing about how cute your characters are? Tell people - and post pictures. People are more likely to be interested in what you're doing if you're enthusiastic about it.
3.) Be positive about your work. Don't EVER post a picture and tell people "I know it's not great, but..." or some variation thereof. If you don't act like you like it, why should anyone else?
4.) Participate in community events/happenings. Twitter is a great place for webcomic-artists, I've found, and there are a bunch of weekly webcomic-events to participate in - #webcomicchat and #ComicTalk being the most popular, AFAIK.
5.) Be patient and keep at it. Building a presence and promoting your work is a marathon, not a sprint. There's no magic recipe for instant success, because "instant success" doesn't really exist. Most of those overnight sensations you hear about? They've been working on their thing for a long time, and the only reason it seems like an overnight success is because you're only just hearing about it.