One of the biggest things that you can do to cultivate a good relationship is uphold your end of the lease. If it is written that you are to take care of the repairs in a timely manner, then do so. If your tenant states that something is wrong and they would like it fixed, then do what you can to make that happen. If you are supposed to come around in person every month to collect the rent, stick to that as much as possible. Always do what is expected of you as per your original agreement and that is a wonderful place to start.
Another thing that is extremely important in having a solid relationship with your tenant is to go the extra mile. Now this doesn't mean you need to come around to their apartment and cook them dinner every Friday, but a message every now and then simply checking in can do a great deal to help your relationship. Ask if everything is going okay. Check if there is anything you can do or help with in regard to the apartment. If they respond that there is, then help them. One of the worst things you can do as a landlord is ask if they need help and then tell them to do it themselves.
Speaking of the professional relationship, this is one of the biggest things. You should always keep the relationship between yourself and your tenant professional. You could be renting to your best friend, but business is business. It is much easier to let one's friends get away with things than people you don't have a personal connection to. When it comes to property management, there really should be a separation. The relationship between you and your client is extremely important, so you want to do everything you can to strengthen it, even if that means treating it the same as any other work relationship.