It's a few years since I was involved in this industry but it's still true, the basics of it. The value of any marketing campaign depends upon how tightly you target your advertising. There's no point in your trying to advertise cars either to non-drivers or those too poor to afford one, no point in advertising diapers to those without children.
So the start of any campaign has to be the aquisition of a target list, for example, a marketing list. I recall one campaign we ran where our targets were solicitors (one of the two types of lawyer in England). The customer expected us to use the phone book and simply call every advert in it....we argued that to be a solicitor you had to, by law, be a member of the Law Society. So we shoud use the Law Society membership list, which is what we actually did.
Now not every list of mailing leads is as easy to discover as that: which is why for most such you'll probably want to go to a list broker, they collect and collate these for a specialty.
For example, there's no simple way of collating your own mortgage mailing list so why not click through any of the links and see what the specialists can do for you?