We've found a few things in our letter campaigns that seem to increase the response rate:
1. Keep the message short and sweet. Don't lie about your situation, but don't give all the details either. A simple message stating your name and that you are interested in purchasing their property will suffice (similar to what is above). This will leave them intrigued and may get them to call where you can explain the situation that you are an investor.
2. We have tested several different ways of sending...from me, from my wife, and from both of us. By a good margin the best response we get is when the letters come from my wife. I think it's probably just less intimidating for a homeowner to call a woman back than a man.
3. Hand write the letters or pay to have them hand written.
4. Hand write the envelopes or pay to have them hand written.
5. Include a return address.
6. Use decorative stamps rather than just a standard stamp.
Essentially, you are trying to build intrigue with the homeowner to get them to open the piece of mail rather than throwing it in the circular file...this is the reason for the decorative stamp, handwritten address and return address. Then you want to keep them intrigued with your vague message. All of this adds up to a better response rate and more calls.
Now, like Tony said above, most of the calls you receive will be a waste of time, but it's a numbers game, and if done consistently you will get deals.