A letter should be as long as necessary and always interesting to the reader. Develop the fine art of faceting. It's the facets that give a diamond its sparkle and appeal. It's the same with a business. Find different ways to tell your story and look for new, unique angles or facets of the business, product or service.
Nothing is easier to ignore faster than a boring business.
Focus on why customers should break down the doors to do business with you. Dig deeper and get at the valid reasons why you deserve the customers' business. Develop a sense of excitement. There is little or no action without a sense of urgency. Certainly sales are few and far between if there is no urgency for the customer to place the order.