There is nothing on earth that you can not have once you have mentally accepted the fact that you can have it.
Constant repetition carries conviction.
A single idea - the sudden flash of a thought - may be worth a million dollars.
The mere fact that you have obstacles to overcome is in your favor.
If you don't make things happen then things will happen to you.
Study your reader first - your product second. If you understand his reactions, and present those phases of your product that relate to his needs, then you cannot help but write a good letter.