"Investigate" any tenant you are considering. A bad tenant can cause extensive damage, leave with months of unpaid rent and you will be left holding the bag. Always get references and do background checks on anyone you might accept. Do not feel pressured to rent to anyone that you know will not be a good fit.
If you've lost money on an investment, take the time afterwards to understand why it happened. Look at your books and see where you went over-budget. Think about the things that you could have done differently to keep the numbers in check. Since big money is involved here, you need to learn as much as you can from your own mistakes.
Before you make the jump into investing in property that will depend on rent for a return, it is recommended to perform a background and credit check on the current renters. You need to understand that individuals with a shady background will more than likely not take care of what is not theirs and damage your investment property physically as well as harming the assessed value. Though background checks are not always accurate, they can go a long way toward mitigating risk.