I know there are a lot of facets to this question and I would like to hear from a variety of perspectives from the many experienced investors on here.

Sometimes a bad run can happen on 1 or 2 properties so I always stress the importance of building a large portfolio especially when investing in sub $100,000 B or C class properties.The likelihood of 1 or 2 properties being vacant within such a portfolio is quite high so it's vital to own 5-6-7+ properties.While the others are performing, some won't and at the end of the day it will level out the portfolio for a decent cashflow return. 

That said, if you have a particular facet of your process that is critical for other aspects of your business, your bookkeeper needs to be aware of this so they can be mindful of that as they go about the work.I do both tax prep and bookkeeping - I'm an enrolled agent on the tax side, but I also do all of the bookkeeping work for a CPA and have a few bookkeeping clients of my own (including some that I also do the tax returns for).

As @Chris Seveney said, this game has many facets....and appreciation is one of the most powerful.

What I am seeing around with occupancy is two facets, the legal IRS/fannie mae/freddie mac rules and also the intent.

You'll realize why coming in with a position of strength is vital-- if you cant practice such discipline, the realities of the market will make you.

Understanding the local market there, like rental rates and property trends, is crucial but hard to do from far away.Finding a reliable property manager is vital for tasks like tenant screening and repairs.

While a good list helps, talking openly with tenants about property expectations is also vital to prevent unnecessary damage.

Renewal Fees are a vital part of a Property Management agreement. 

Street lights, pot hole maintenance, water quality, sewers and hospitals... are vital to safety and the value of your real estate.