Make an offer. You’re not providing information–you’re actively selling. So you need to present the reason for sending your letter. There is something new about your product. You’re writing to people who attended a particular show and are offering a discount. You’re giving a second product free to people who order a first product. “Order five and get a sixth free.” “Join for a year and get two months free.” “For a limited time, respondents can take advantage of a one-time trial offer.” “Book the service now and get a discount for summer delivery.” These are all extra enticements designed to push the prospect over the edge and get him or her to respond.
Repeat the offer. State the offer at least twice in your letter, at the opening and at the close.
Add a sense of urgency. Give the prospect a reason to reply right away. If you can’t persuade the person to act while he or she is holding your mailing package, chances are the person will never respond. So add a time deadline for a response. Or an offer on top of the offer for a quick response. Or maybe a limited amount of the offered product is available at this special price.