The most important thing could be summarized in three basic things that are the pillars to have a healthy and lasting relationship.
Communication: It is fundamental to be able to communicate with the other person in a totally open and honest way. But of course, for that the second point of view must be integrated and respected. As much as we do not like what our partner tells us, we should never disrespect.
Respect: We have to take care of our words. A father told his son that every time he said a bad word someone had to put a nail in the door of his room. When a month passed, the father told his son that he now had to remove all the nails from the door. Once he finished with it, he saw that every nail, no matter how small, had left a mark on the door that was very difficult to remove. So his father looked at him and told him that before saying anything it was necessary to think if those words were correct because they would leave a mark in the heart of that person. So whenever you want to communicate with your partner and say something important take care of your words, never hide anything however hard it is but do it with tact.
Put effort: a relationship as a couple, as a plant, as a house, as a car, as a boat, as a friendship, as everything in this life, requires your time and dedication. Maybe a few days more than others but never take it for granted. With a nice message, nice words, a small detail like a flower, a postcard, buy your favorite sweet, little things that make the other person we care and we are there.
With these three simple but important things I assure you that you will have the perfect base for your relationship to be healthy and lasting.