Well for one I get my list form a company that focuses on providing foreclosure information. Second, I hand write everything and make sure to use real stamps not printed or bulk mail. The trick is getting the recipient to receive the message. If they don't open the envelope they won't get the message.
Envelope selection matters too. These people are delinquent. They receive collection notices from their lender and probably their car loan, credit cards, and who knows what else. The last thing they want to open up is a letter that even remotely resembles a bill or collection letter. They don't want to be sold either so post cards and fliers are another no no.
Also, don't write a 1,000 word essay about how great you are and how you can help them. Be straight and to the point. Either they're ready to sell or they're not.
I use to only send out 100 mailers per week and got an average 5-10% response rate. I'm sending out 100 pieces tomorrow. Let's see how that goes.
These are my personal results and I don't claim that they will always work or that it will work for everyone but it's worked for me in the past and if it ain't broke...