I really should take some classes in marketing, but until then, I'll keep on trying to learn from the wisdom of others. Like this post by Erica Friedman about the difficulties of advertising and promoting manga. I don't write manga, obviously, but the concepts apply to anything you're trying to sell.
The core of advertising is saturation. One of the tenets of advertising is that repetition is the key. The ad for XYZ car may annoy the heck out of you, when you see it on TV, hear it on radio, see it in a magazine and on billboards, but chances are, you'd recognize the car if I showed you a picture. Buying one ad won't make a difference. A company has to buy many, many ads to establish in our thick brains that a series is out.Saturation as the core of advertising isn't her point at all, by the way. Her point is that even after you spend a ton of money saturating websites and print publications with your ads, no one's actually going to pay attention to them. Depressing, but true.
The alternative, however, isn't to not try to saturate. You still have to get the word out. How to do that effectively (both in terms of cost and results) is the hard part.