I've been doing direct mail for more than 25 years now. Statistically speaking, more than 80% of your deals will come at or beyond the 5th mailing. Does that mean you won't get a deal from the 1st mailing? Absolutely not. It means that you will get momentum around that time, (month 5) if you mail every month. That's the key. Consistency.
Here's another statistic for you; most people will quit mailing on or around the 3rd mailing. You want to be the last man standing when it comes to direct mail. Just by staying in the game you will be ahead of most people.
I use over sized postcards for all my mailings except probates. I only use white computer generated letters for those. Hands down they have worked the best for me. I have specialized in probates for more than a decade.
Direct mail results are sitting at about a 1% response in most areas around the country. To put that into perspective, on average, you will need to talk to 15-20 perspective sellers before you get a deal in most markets. If you are mailing 1000 direct mail pieces a month that means you will only get about 10 calls.
So you will either need to send more mail or develop leads from other sources like driving for dollars, your website, bandit signs etc. You might also consider adding ringless voicemail to your direct mail campaigns.
I hope this is helpful.
Sharon