A lock box is your friend especially if you do this on the side and have a full time job.
It's a simple little thing that has combination and hangs on a doorknob. The combination opens a little compartment that you can store a key in. Investors can go by and check out the property anytime whether you are there or not! 
One thing to remember. When you give this combination out make sure you know or trust the person going in there. I can usually tell if I have a player on the phone or not. Only major players get access to my properties without my presence. I have an obligation to protect the house. One thing we don't want is to come show a house one day and all the copper be stripped out because I gave to lock box combo to an untrustworthy person.
You can pick one of these up from Lowe's for $30 and it will instantly pay for itself in time/gas savings.
Own Your Situation,
Ben
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