More than in any other form of advertising, in direct response advertising, the copy is the defining and driving force, not the graphics. The graphics are there to support the copy.
 

Copy Guidelines

• Know the product, all the selling features and benefits.
• Know the market.
• Talk to the prospect in their language.
• Make a promise, then prove that you can deliver on it.
• Get to the point right away.Make the copy germane to the selling proposition.
• Make the copy concise.
• Make the copy logical and clear.
• Make the copy enthusiastic about what you’re selling.
• Make the copy complete, answer all the questions.
• Make sure the copy sells.

21 Copy Rules

• Use short, sentences as much as possible.
• Avoid semi-colons-they slow the reader down. Dashes and elipses separate complex or long thoughts yet carry the reader onward. (Reading should be made as easy and rapid as possible.)
• Keep copy in the active tense-complex tense structures make copy boring and hard to follow.
• State the price and offer on all package elements unless they are being tested. 
• If available, include testimonials. They are an unbeatable assurance. Names are better than initials; specific comments on specific aspects of the product are better than general praise; "results" are more powerful than opinions.
• Specifics are always more effective than generalities. Concentrate on examples, titles, names, even quotes. Position product benefits as reader benefits-tell the reader what's in it for him.
• Follow the "rule of three"- series of three has more rhythm and balance than two or four examples or adjectives.
• Odd numbers (7 reasons why, 21 basic rules) are more effective than even numbers.
• Always seek a rhythm in copy-it should "sing." Read your copy out loud or have someone read it to you to be sure it reads the wayyou hear it in your mind.
• Whenever someone has to reread a sentence or ask for clarification, change the copy-it will bother a substantial portion of the audience as well.
• Suit imagery and vocabulary to the market and the product. If you are selling a magazine, for instance, the copy should reflect the style of the magazine.
• The headlines, subheads, boxes, photo captions and sunbursts in a brochure should be a full sales presentation for non-readers.
• Underlines, indents and the use of second color in the letter should be used for pacing (to break up the copy) and to make all the key sales points stand out clearly to the prospect who only skims.
• Never ask a reader a question in a key headline or on the outer envelope that can be answered, "No I don't want this" or "I don't care."
• The letters in a package should be personal and look like letters . . . with typewriter type, a salutation and signature. Use "I" and "you“ (lots of "you's").
• The first paragraph of a letter should be no more than 1 or 2 lines, 3 at most, to make it easy to begin reading.
• At least the first page of the letter should "break" to the next page in mid-sentence, preferably at a point which pulls the reader onward. For example, "The small child ran directly in front of the speeding car and . . .“
• Mention the product on page one of the letter, include price and offer if either is a key selling point. If the letter has a "story" opening, consider a "preface" above the salutation to state the offer.
• A postscript is one of the most-read portions of a letter. Use it to reinforce the sales pitch and stress the incentive for immediate response.
• In concluding the letter, return to the theme that began it.
• The elements of a direct mail package do not all have to be "themed" together, but there should be a sense of continuity-a feeling that everything in the envelope was not collated from a random assortment. The idea presented on the outer envelope must be developed either in the letter or the brochure and this main theme is usually mentioned on the order form (which in addition to presenting full offer details and a contract of sale sells by highlighting the key incentive for response).