One of my favorite small business lead generation strategies is to define a mailing list for a target market as narrowly as possible, with the intent of getting that list to a very small number. Once you do this, knowing that most anyone on the list is a good suspect, you can possibly more easily afford to mail to this list multiple times.
This strategy assumes that you have some goals in mind in terms of how many leads and clients you need to generate from your marketing activities.
If you only need 10-12 new clients this quarter, then you are looking to carve out a list of maybe 500 very defined members a mailing list. If you want 100 leads a month, your list might need to be 5000. The point though is to work hard on gaining the trust of that narrow list instead of sprinkling a little bit of marketing dust on the universe.
The somewhat hard part of this equation can be finding a good way to narrow your list. Most direct marketing pros will tell you to work with a list broker. A list broker is someone who sorts and resells lists from various publications and organizations. In some cases you could just go get this list from the owner, but a good list broker should have some history on lists that work, how to get the best deal and where to find the best list for your specific needs. Like any resource the trick is to find a good broker.
Bob Bly, author and copywriting genius has a list of list brokers that he uses and recommends. This would be a good place to start your search. Before you pick up the phone and contact a broker make sure that you can tell them what your marketing goals are, what you intent to send and, most importantly, as much detail as possible about your target prospect.