Keeping quality, long-term tenants takes a lot of the stress out of managing your properties. Some you will have no control over. A job will cause them to move or any other number of life events could cause them to move, but there are certain things that you, as the landlord, can do to try to keep those tenants a bit longer. Even if they don't always stay, these tips will help improve their review of their stay in your property and that helps to fill the vacancy quicker.
The first rule to keeping good tenants in your properties is to be up front with them about any and all rules that you have for the property that they'll be living in. You don't want to spring anything on them after the lease is signed or it may - and very likely will - break their trust in you. Explain all policies before they sign their lease. A good practice is to provide a copy of these policies to them.
Response time is key. You may have multiple tenants and of course you do have other responsibilities outside of land lording as well, but that does not mean that a question or complaint should sit unanswered for days at a time. Try to respond within 24 hours and you'll be sure to earn loyalty from your tenants. In the same way, reacting to maintenance issues is highly important. You should respond to complaints promptly and handle them as quickly as possible.
Incentives are also a good way to obtain and keep the best tenants. It can be something small, like a discount to a local gym or covered parking, but it will be something to help you stand out against the rest. The beginning and the end of the lease term are the best times to offer these.