The best way to measure advertising results is to track responses from various media.  You can code your print advertising and direct mail, for example, by giving customers a specific code to use when responding by mail.  So, let's say you're advertising in the January issue of Esquire magazine, you might ask respondents to use code ES -- 1. For radio, television, and other media, such as billboards, you can track your responses by using different toll-free numbers for each medium or campaign.

Also, make it a standard practice to ask all prospective customers where they learned about you.  Develop a lead sheet that includes all the possible sources and supply them to the members of your staff who answer the telephone and your sales team.  Make it a requirement to complete this lead sheet for every prospect. 

If your new customers make purchases on your website, be sure to include a question regarding how they heard about you in the checkout process.  And, depending on the type of business you operate, you may also be able to ask new customers the same question via a postcard or short survey following a sale.