The name junk mail is just that for a reason; because it's junk. A company may have many wonderful things to inform you about but unless they grab you and give you a reason to read, you will find yourself dumping it without giving it a second glance. Don't let this happen to your letter.
Start by making sure you know your audience. Sometimes hitting the masses and hoping that your target it nestled somewhere within is not as effective as shaving down your list knowing full-well that these recipients are people who will be interested. If you plan to do a mailing within your client list, it's an easier process. If you go outside the confines of your own client base, it's important to try to go through your lists and figure out exactly whom these people are. If you haven't taken the time to get to know them, why should they want to know you?
Get a strong writer to write your copy. If you don't already have a person dedicated to marketing, find a writer you feel comfortable working with in getting your message across.
So now that you have a person dedicated to writing your letter, you're going to want to make sure all the informational bases are covered within the content of what you'll be sending out. Try to approach the information as your client would and make sure the following things are addressed:
The first paragraph should identify your product/service and describe how it will benefit the reader.
The second paragraph can expand on the benefit.
The third paragraph should describe the features of the service.
The fourth paragraph is a great place to include testimonials if you have them.
The fifth paragraph is the call to action.
Keep the language simple and don't get too deeply entrenched in technical terms. You want to draw the prospect in, not confuse them. And don't get too concerned if the letter is more than a page. If you have grabbed your reader's attention and need to finish your message up on a second page, you will be safe with that.