I own a few rental properties as well, thinking it would be a good source of income for my retirement years.
You quickly learn to carefully screen tenents, or you will loose money in an awfull hurry. Welfare scumbags, and others with no ambition in life to better themselves do not care about other peoples property. they will smash it up and just move on to the next place.
Those people deserve to live in rat holes, because they are the ones who created them in the first place.
Best unsolicited advice I ever got: ask for the previous 12 months' cancelled rent checks from prospective tenants.