I usually use webcomicavenue, promotecomics, supportcomics and webcomicnetwork. Promote and support are almost instant in their retweets O_O
I also suggest not just following people that promote, but other creators. Just following and liking them help (helps them and helps you). People can see your comments and likes and might check you out. I know that has help me, of course do it cause you like the creator or twitter person, not just for the idea that it might get you more exposure.