[jsackmann]

Maybe this is obvious, but in case it's not: For those interested in really studying direct mail tactics, there are lots of books on the subject by experienced practitioners. I read several of them a few years ago; wish I could highlight one or two as a recommendation, but I don't remember.

As noted in some of the other comments, these folks test everything, right down to the color of the envelope and the size of the signature at the bottom of the letter. It's also fascinating to read war stories. IIRC, there's a direct mail letter for Reader's Digest (or some similar publication) that was most effective for decades. They kept testing it against other contenders, but that same letter kept getting results.