@Brook Wilson just like @Ayman Elmasik said, start working on your Buyers List. I personally don't think you need to have a 25,000 list with people from Mars listed as "cash buyers" but you will need to have a solid list. Yes, everybody says find the deal and the buyer will show up. But the reality is that it isn't that easy.
We have had to build our list slowly and with a lot of effort but our list is composed of true buyers that either have added themselves to our cash buyers list by visiting our landing page website or people we have personally met and asked us to include them on our list. You can always ask around the local REIAs and some people would offer their 37,950 buyers list for only $997 but to me that is the "easy way" and probably those addresses came from a car dealership or McDonalds or god know where. I won't sell my list and I won't buy anyone's list (period).
Regarding your question about frequency of mailers, every 4 weeks works for most of our clients. Some that are doing a lot of volume send every week but to different areas in order to keep their acquisitions team busy with calls and appointments every week. But that is long run when you get to send 10,000 - 20,000 letters every month.
Good luck with your business.