Have you ever thought about sending out postcards instead?
You need something catchy to grab their attention.
One of the members here posted a picture of their postcard they designed, and IMO it looks pretty darn good. I'm not suggesting for you to plagarize his work, but you could use it for ideas.
Here is the thread I'm talking about.
Letters to me seem pretty boring. If the letter is too long, will the homeowner take the time to read it? Some may argue that if they are looking for lawncare they will, but I still think it's better to keep your message short and sweet.