To answer your question, the majority of marketers here will tell you Direct Mail is the way to go.

I am on several mailing lists and maybe 95% of what I see is not good (inflated ARV, underestimated repairs, etc). 

Does your group do direct mail or online marketing (PPC, SEO)?  

I did send them acknowledgemention the mail stating they chose to break the lease and I was not returning the security and cited the reasons why.

Every couple of weeks, I get a direct mail attempt to buy a property I purchased from a bank a few years back at a very very good price.

That is the question:) We find ours through direct mail (costly but still effective), craigslist (learn how to weed through the junk), and our website with FB ads.

I have asked the tenant about other people living there and reminded her that any additional adults living there would cause an additional $150/month in rent.

I do not have direct plans with them right now but I watch their webinars and am on their mailing lists because I have talked to them and reviewed their activity.  

Hey everyone, I am working on a few different direct mail campaigns and am wondering if anyone has any tips or systems they use to make sure they are not mailing the same people on more than one campaign?

What we do is document all of those activities in our pipe drive system so it notifies us with reminders to follow up.