The way we keep track of all the invoices and which property they are for is pretty simple.  

http://www.loopnet.com/Listing/20073204/1286-N-Los-Robles-Ave-Pasadena-CA/Dang, that one does look sweet with the zoning for 5 additional units.

But unless you're an expert bookkeeper/accountant, you won't set up all the extras, like bills and invoices and a proper chart of accounts, that lead to answers to the questions folks like you and I really have:How much money is owed to my business right now?

Biggest downside is it's kind of a sleepy city compared to some you mention on your list and it gets dang cold and stays cold pretty long.

Dang! 

I used to just pay invoices and think it was a waste of time to shop around too much. 

@Michael Dang  My understanding of the Vacancy / Loss is money not earned while a unit is not rented, between tenants, or if it is off the market for some time for some reason (Vacancy), and Rent due but not collected (Loss).  

Dang Elbert, go you.

I later got billed a single invoice for $850 for repairs plus $150 in management fees without ever authorizing any of the work.  

Regardless of how you do it, you need to really be on top of bids, invoices and payments.