Budget for all of the direct and indirect expenses.Be comfortable hiring others to supplement what you do.  

My wife and I are discussing buying our first rental property with the aspirations of having 3 or 4 to supplement our current income and hopefully fund college for our children which are still young.Our plan was to buy a home at 50-75k and rehab it with 20-25k and then rent it for 1-1.2k.

At the moment I'm looking to supplement my income.

Meanwhile the other tenants will be helping pay your loan (ideally partially supplementing the cost on your unit).  

Passive income stream to supplement current retirement income.2.

I am a teacher and have two fellow educators who plan to working together to flip a house this summer to supplement our income in the short term with long plans of getting out of education sooner than later.

I would ideally like to transition fully into real estate investing in the next year, possibly by supplementing with a few traditional real estate deals once I pass my real estate licensing exam and get sponsored by a broker.

If you just want a half a dozen free-and-clear rental properties to supplement your retirement income, then pay them off (especially if deals are hard to come by or the numbers don't make sense).There are many people on BP that haven't been in the real estate game long enough to have weathered the 2007-2008 storm with substantial real estate holdings, so be leery of taking advice from these people when it comes to risk and leverage.  

Their printed supplemental materials were also top notch.

He is even supplementing the deal with a cash supplement of $1,050,000/ yr until the debt is due in full June 2018.