(Excerpt from Breakthrough Advertising by Eugene Schwartz)
If your prospect is aware of your product and has realized it can satisfy his desire, your headline starts with the product.
If he is not aware of your product, but only of the desire itself, your headline starts with the desire.
If he is not yet aware of what he really seeks, but is concerned with the general problem, your headline starts with the problem and crystallizes it into a specific need.