As someone who's been (and still kinda is) in your shoes, the best way to self promote is take part in forum discussion. Comic sharing threads are a great way to help connect with fellow creators on your level especially starting out. I've also found other solid ways to build your sub count such as checking out the "fresh" section of Tapastic for example. Often, I've checked out comics that were just starting and if I liked what the author was pitching, I'd like and subscribe to his/her comic as a way to help encourage them. Posting comments that actually relate to what's going on in said comic(s) goes a long way towards establishing yourself as a fellow creator without coming off as desperate for attention. Lastly, I've also clicked on an author's sub count number and from there have searched for creators I may have overlooked and started up a friendly conversation with them. Sometimes, they ignored me. Other times, I had a new sub or two before I knew it.
Oh and while I'm on the subject, keeping to a consistent update schedule for your webcomic can in the long run help you gain new subscribers passively speaking. That is, comics that get a fair amount of attention on a regular basis have a chance of showing up in both the "trending" as well as "popular" sections of the site which of course helps you as a creator get more attention for your work without having to ask. 
