And then I would surprise my tenant with something nice for them, thank you letter and gift certificate for dinner, something just to say thanks for working through things to assure on-time rent payments (yes, VERY good business, it's called CONDITIONING).

A letter from an attorney quoting the appropriate law and CFPB regulations may be necessary, with additional follow ups.

Would sending out “prospecting” letters be worth it?

Things I learned in my first job that have continued to prove valuable today: underwriting, managerial accounting, private equity deal structures, what a side letter is, what investors want out of deals, what a K-1 is, why real estate is a valuable investment class, what asset allocation is, where to find tenants, what cam is, how to handle Capex items in sophisticated underwritings, loan docs and negotiating terms thereof, understanding a lease, what an estoppel is, what a phase one and phase two are, how to read a PCA, basics of land use and zoning...Oh and best of all, I got paid to learn all of this stuff.  

However, my perspective is limited, so I thought it'd be fun to start a conversation comparing the tried and true with the new kid on the block in this changing world. 

My partner and I are going to send out handwritten letters to home owners in our nearby city.

You can do yellow letters. 

BTW, we love this market because we've got a major university a few blocks away, Fortune 500 employers in the area and other factors which drive constant rental demand.

All areas of these zip codes are not quite the same---things can change pretty drastically block to block and as you move further east past Hwy 71.

Follow up question:  I see a lot of articles mention that "the property is located in a B- neighborhood" what does the letter grade of different neighborhoods mean?