ARV and Rehab costs are where BRRRR investors tend to make mistakes the most.

A PMC is the ONLY one that will deal daily with your property.Here's some copy & paste advice on hwo to screen PMCs:Recommend exploring as many sources as possible to get referrals AND cross-reference them to get as much accurate information as possible.Check out NARPM.com, BP’s Property Manager Finder (BiggerPockets: The Real Estate Investing Social Network), etc.Also, encourage you to learn from the mistakes of others - by reading posts here on BiggerPockets about owners not having their expectations met by their current Property Management Company.

Run realistic numbers and be prepared for the worst.

@Cole Starin sometimes it makes sense to sell the WORST property in your portfolio.Another option is why do a LOC on the duplex, when you could just do a $100k cashout refi to pay off all your debt AND keep the property?

I am guessing you like the numbers you see in Strawberry Mansion and Norristown (Montco not Philly) but do you know for a fact that at those numbers, you can do a full gut rehab and be at a number that will allow you to profit, and along the way make some sizable mistakes especially if this is your first project . . . 

You can hear real-world examples of how others have built their investment portfolio and (hopefully) learn to avoid their mistakes.4.

You really have to anticipate the worst and fix it before it happens.

.-- Or, what we do is offer the free rent AFTER & ONLY if they pay their first 3 months of rent on time to earn it.4) Lower the rent 5% every 2-4 weeks until activity picks up.5) Understand this is the worst time of year to find tenants.-- Starts picking back up end of February when this tenant pool starts getting their income tax refunds and aren't broke anymore.6) Section 8: where is the property being advertised?

A common mistake in my area is to ignore Hotel Occupancy Tax.

This can be very expensive and is usually the worst choice because you can't justify the cost.2.