How do you reduce the risk of purchase in print advertising?

One way is to ask publications if you can advertise on a "Per-Inquiry" basis. In Per-Inquiry advertising, you pay for print advertising based only on the responses your ad generates. Some companies pay a fixed amount for each call the ad generates, some pay based on each sale the ad generates. Per-Inquiry advertising is common in direct-response radio and other mediums, but is relatively uncommon in print so far.