I'd have to agree that soft sales tactics of the kind where the customer is expected to do much isn't going to help, that more than likely it's going to involve leg work and at least some phone calls.
You could, however, send out the letters...
And then a week or two later follow up.
But you have to follow up, and be ready for answers such as "WHO?!"
From the folks who, letter and all, still haven't a clue as to who you might be.
It still might open some doors, just be aware, like they're saying, letter alone's not enough.