First, you decide who you want to reach.
Second, what's the best way to reach them?
Do you attain the best results by calling them, sending a fax, or mailing them a postcard or letter? You can greatly increase results by using a combination of these methods. Such as, sending a letter then in a few days following up with a telephone call.
Last, how much are you willing to spend?
Decide what is your budget and what type of response you will need to have a successful mailing. A typical response is between 0.05% and 2%. Also, keep in mind, that it takes several mailings to make an impression, so you should mail on a regular basis.