I am primarily interested in buy-and-hold investing for cash flow purposes.  While I certainly hope to realize decent appreciation over time, I want to quit my day job sooner rather than later and need to replace at least some of my current income in the near term.  

However, as I poke around and talk to folks, it seems like per-unit cash flows can be pretty modest - sometimes only $50/mo or $100/mo per door.  While this can become a sizeable income once you have 25 or 50 units, I'm curious what type of investment properties or strategies typically generate the highest cash flow per unit/door.  I want to replace my income over time, but ideally in bigger steps that $100 per door.  (Note I am interested in the actual dollar amounts, not percentages - $500/mo take-home might be only 10% of a $5000/mo rent, but it's preferable to the $300/mo that is 30% of a $1000/mo rent.)

What type of properties tend to have the highest cash flow per unit?  What investment strategies or tactics are most likely to maximize cash flow per door?  And are there geographies or markets that tend to produce the highest cash flow per unit?  Finally, if you have any recommended reading (blogs, books, etc) on how to invest in order to maximize cash flow per unit, I would be grateful for the pointers.