Respond quickly to tenants on work orders that you are responsible for. If the roof leaks call your roofer and keep the tenant in the loop on progress. If the AC is out and you are responsible then do the same. It is just like any other business. Great customer service will keep them.
Pay your rent on time! It is no different than having a high credit score and having great leverage with your bank. If you have an excellent track history with your landlord, you are much more likely to get a favorable deal.