Try to lead off sentences and phrases with benefits. Write in short sentences. Use words of one syllable as much as possible. Don't assume that the person you're writing to is as literate as you are. Even if he is, he's distracted, and he's trying to extract the key information he needs, often by just scanning your letter.
Clarity is more important than literary merit.
The ability to sell is more important than the ability to write. When you've fully described the many ways your product will benefit the reader, show the reader how to acquire this fabulous program. Spell out your offer in detail. If you're offering a premium, this is the place to sell that a bit, too. You may also feature it in the brochure, if you have one, or in a separate premium flyer.