I think the key is a drip mail kind of thing. Where something gets sent out every few weeks or so to the same person. "They" say the first two will get tossed, the third will get a glance, and then tossed, the fourth and fifth you start to get their attention. BUT, that too, is expensive.
Mailings definitely work, you'll get calls. Start small, have a goal of getting out at least 25 a day. Mine are a full page sheet of paper, envelope, stamp, etc...I write, stuff, and stamp them myself. I've gotten a very good response from them. For every 100 sent, I'm getting 5-7 calls. The only thing I've tested with was the color of the paper. Different shades of green and yellow. Don't know what it is, but I get a better response from the yellow shades.
I'm also considering adding a buyers list invite. Some people might not want to sell at the present time, but who knows, they might be looking for more property. Kinda hit 'em on both with one letter. I mail to mainly OOT of SFH and Multi-family. I cross-reference their names and address from the Local Tax Assessors office.
As far as the content. It conveys the message that it can be stressful to own property in another City or part of the county. And if they have a property that has become more of a liability than an asset, we might be interested in buying. I give them a brief explanation of what I do, and if they had an interest to call the numbers listed.
Hope this helps