If you promote something twice to one hundred people it will lead to more sales than if you promote it once to two hundred people.
Frequency galvanizes attention and improves trust. (At least one kind of trust).
The curse, of course, is that the best members of your audience, the ones who are listening the most carefully, have to be bored/annoyed at the messages that show up after they take action. Some people pledge the first day of pledge week, or buy the book the day it comes out. Those folks don't want or need to hear the message again.
Worse, frequency creates a culture of less engagement. Since we know that just about every important issue, opportunity or warning is going to be repeated a few times, we don't engage as much. Why bother to listen, we say, they'll just repeat it.
The line between frequency and annoying is thin indeed. You believe in what you sell or you wouldn't make it, wouldn't devote yourself to it, wouldn't sell it. At some point, though, the frequency of repetition stops being helpful enthusiasm and starts being selfish. And, alas, there's no easy formula available. Jay Levinson likes to say that you should change your marketing not when the staff, your family or your agency tells you to--but when your accountant does.
I'll confess that I'm bad at this. I'm okay with gleefully saying, "I made this," but not so good at saying it two or three or yes, 19 more times.
I'd like to tell you that there's a magic solution to not repeating yourself as a marketer, to respecting the best and brightest of your tribe and being able to merely whisper about your new project. This approach works great if you focus on creating scarce goods (popular fine artists, jazz stars and small restaurants don't need to spend a lot of time reminding people about their soon to be sold out goods), but most of the time, as you work to reach the edges, frequency works.
Curse or not, the fact remains: frequency works. We're going to be stuck with it for a while, I fear.