William, the "secret" is a combination of 3 things:
1. Message
2. Medium
3. Market
Message: The more heavily it screens sellers the fewer calls you get. I send all of my calls to voicemail or website so my goal is to generate the highest response possible and let Step 2 of the marketing process screen them.
With that in mind I use the headline: House still not sold? Just about every seller feels that their house hasn't sold fast enough so this doesn't really screen anyone out, BUT it does tug at their frustration. I find the body of the message should be as short as possible. I've even sent out postcards that only had the headline and my phone number and they work well. But when I include some text it's usually informal, like I'm just a guy who's interested in their house.
Medium: It's postcards hands-down for me. I get 3 times as many calls from postcards as I do from letters. Bright yellow cardstock with black text in Courier New font works well for me. Although with the prices you can get full color glossy postcards printed for these days, I might try that in the near future.
Market: This is probably the most important component. I market to active MLS listings. I'm not trying to cut the agent out of their commission, afterall the SELLER is still responsible for that. BUT I've learned through agonizing experience that it's necessary to bypass the agent to get the offer presented properly (or at all).