Direct Mail

I do not find direct mail to be successful for my agency. However, I have had one large event where we received 300/7500 pieces back. The piece was a FREE VIP coupon, and it worked. However, normally I see only 7 calls per week from direct mail, and that is after sending out about 15,000 pieces per month. If, you found direct mail to work for you, please share why you think it is successful.