Communicate with each other. You may not always agree, but by constantly knowing where the other stands, you will eliminate any faulty assumptions. You're only human, and it is inevitable that conflicts will arise. Espouse an open door policy and don't get defensive. Resolve to solve your problems together.
Give each other space. There is merit to the idea of two becoming one, but ultimately you still are two distinct individuals. Respect your differences and don't try to change the other. A relationship needs to breathe in order to thrive.
Maintain your sense of humor, as it can be your ally during the tough times. Granted, you don't want to use it as a substitute for dealing with an issue, but it does help in keeping things in perspective.
Nurture your sex life. In the beginning, everything comes naturally as the newness and passion seem to take care of themselves with little effort. In time, however, it too needs work just like the overall marriage itself. Don't be afraid to talk about desires, needs and concerns or even going to a therapist if need be.
Appreciate what you have by not taking each other for granted. No matter how long you have been together, you should never make a habit out of assuming that the other knows how you feel. Even if your spouse does, affirmation is a great way to keep the romance alive.