1) Interest -- Your copy will be interesting if it is relevant and meaningful. To that end write to the demographics and context of your targeted reader. Make it meaningful by expressing the importance of your message in finite terms: this offer will affect the reader how, when, and why.
2) Desire -- It's not about flames of passion, but close. Your words need to satisfy and control an emotional driver: guilt, fear, ego, need for approval, greed, and pride. They need to support a positive decision to act.
3) Thank You Notes -- Thank you notes are scarcer than hens' teeth. Why? Because someone decided the rules of good etiquette can be observed with an email Thank You. Not really. While marketers resort to the less costly email to say thanks, the thrill is gone. If the email includes a comeback coupon, it has to be clicked, viewed, logged and printed. The consumer ends up with an 8-1/2 x 11 paper voucher to file on their dresser beside the bills. By comparison, a mailed Thank You note with wallet-sized coupon card is convenient to the consumer. It's classier too.