you should wait until you have at least a small archive of strips before you start advertising your comic, whether you're buying adspace or just pimping it on a forum, because if you don't have much to look at it's not as easy for people to get hooked or get a good impression of whether or not they like your stuff.
cheap advertising is always nice (you can buy adpace on my site if you want
) another great thing is doing fanart for comics you like. usually they'll post it in a fanart section with a link to your comic and even talk about you in their blog thing. i hear link exchanges are good, but i feel you should only link to comics you think are actually good, as opposed to just linking whoever will link back to you, but that's just a moral problem.
as far as keeping readers, drawing decently well, being entertaining, and having easy site navigation aside... update regularly and when you say you will. if you miss enough updates (and enough may be just one) a reader, even one that likes you, might forget you exist.
that's all i can think of right now, good luck.