It depends on the type and quality of properties you're investing in, the quality of your tenants, and your specific investment strategy.  

Having $2 billion per year instead of $1 billion per year does nothing to improve the quality of your life.

Usually the quality of tenant drops too.

I used Denver Mortgage in the past, but from what I can tell they are either out of business, or the quality has deteriorated very badly.Any recommendations on where to look? 

Eventually I went to check out a place, and while I believe I could get along with the people there, the quality of the apartment was a definite downgrade.

I like the idea behind the ductless systems but am worried about the quality, ease of installation and maintenance.

My goals are to get a flip going within 3 months and to start building a rental property portfolio with quality management companies handling the day to day operations of them.

In order of productivity, based upon lead quality & cost per lead, across over 50 markets in the U.S.:1) AdWords (Google Search) - depends a lot on the keywords you use...2) Direct Mail - mix it up like Cody says, & only mail to people with equity!!

Quality and area? 

It's not a foolproof method of evaluating neighborhoods, but it's a place to start.