When I started freelance, the first agency I worked at was direct mail agency. My experience was limited at that time. Not so now. A half dozen of my regular clients are direct mail and I have over 6 years of direct experience. In a nutshell, it is testing. Great design, great copy and an offer that gets tweaked and retested to see how it pulls. When the winning offer results in a fabulous sales result, that offer gets tweaked again and a new version gets introduced to a new market segment.
While this used to to be referred to as junk mail, there are some gorgeous direct mail solicitations being done. I have been awarded with a Gold CADM (Chicago Association of Direct Marketers) Award for creative for work done at one of my clients. The work pulled fantastically well and it was done very quickly. Direct mail now is a mixture of digital emails and print. This is a more environmentally friendly alternative but the premise is the same... build results!