@Joshua Martin Here's what I do. In google drive, I have a spreadsheet that I enter all new probate leads into, lets call it the master list. This list is synced up with a mail merge google doc, so I can quickly print letters after I add new leads. When I send the first letter, I type in the date the letter was sent into a column on the master list. Over the course of a month, I list homes from this list, people call and say remove me from your list, and houses are listed by other agents in the MLS.......as this happens, I remove these leads from the master list.
Every 4-6 weeks or so, I review the dates of the last time I sent the leads a letter, and if it's been 4-6 weeks since the last letter, I will send another one and type in the date the letter was sent into another column on the master list. Every time I send repeat mailings, the list is getting bigger and it takes me longer to print, stuff, stamp, etc. I think my next repeat mailing will be about 500 pieces. I started this the same as you, getting about 75 leads per month over 2 counties.
I'm thinking about putting an ad on craigslist and paying some $50/day once a month to come in and print, stuff and stamp these repeat mailings. If doing nothing else, they should be able to get that done in one day, and I think I am at that point where I can't do it all myself anymore.