Drip campaigns work wonders!
Here is why. When you send your 1st mailing to an owner, you do not know their motivation level. However if you mail to them between 5 -7 during the decision making process you may mail to them at the time they are really open to your message.
Let me illistrate what I mean. Lets assume you are going after foreclosures. Most people mail one peice when the NOD is filed. Now if the foreclosure process is 9 months long, and you mail to them every 5 weeks the seller has seen your message about 7 times. instead of the other folks who send one or two mail pieces.
Post cards are super cheap. Here is one we use http://www.tonyseverino.com/m/19000/19444/files/Consulting/Forms/Advertising/20-SYHAI_PostIt_Wide_5x3%20V%202.0.pdf
You should consider using a different postcard, however keep the same design, but mix up the message a bit.
When considering drips you should know the decision making period for your list. Expired are different than foreclosures, which are different than tired landlords.
I would recommend that you consider the time frames and then how you can effectively mail to them in that time frame. We use a 6 piece system, the first is a simple yellow letter, and then followed by 5 post cards.
Hope that helps
Tony Severino