If markets and technologies never changed, and all people behaved the same, you could probably write a set of rules and software to do your direct marketing. Luckily that's unlikely to happen anytime soon in this market. If it did, the output from such a process would become boring, and the market would switch to favour the first company which broke those rules. You have to work hard even to stay in the same place in your market's awareness. The expectations of your market change with every day, and with every communication they receive from you, your competitors and from other sources.
Direct marketing is really a black art.
But when all the lights and mirrors are taken away you're looking at trying to affect the behaviour of large numbers of people you will never meet to take a more positive look at a future product which may not even exist yet. The wonder is not that it works so well, but that it works at all. Keep it legal, decent, honest and truthful. This rule is from the Advertising Standards Authority in the United Kingdom. We should always remember that our communications operate within the legal and cultural frameworks within which we operate.