It's tough. I have a long history of shooting videos, so I thought a fun and unique way to promote my mega-rad comic, Satan Ninja 198X, would be to shoot a live-action promo video. Since the comic is a sort of tribute to everything my girlfriend (the comic's artist/co-writer) and I love about the 1980s, we decided to make the video a series of mock '80s commercials, starring ourselves.
Even though the videos turned out totally awesome, they haven't done much for us in the way of attracting new readers. But at least they were fun to make.
It seems especially hard to get noticed as a long-form comic. Gag strip comics can easily get attention on sites like Reddit, but I don't know of any good places to promote a long-form comic.
Unfortunately, just being good, isn't quite enough. Even a great, professional quality comic, like, say, Satan Ninja 198X, can really struggle to find readers. It's a cruel world.