Stay interested in each other. Show you care by using your partner's love language every day. Put intimacy high on your "to do" list. Turn to each other first when you have a problem. Court each other. Do new things together regularly.
I'm a marriage therapist with 20 years experience, and I have witnessed how small choices and decisions on a daily basis can keep your marriage strong, or slowly wear it down. It's the little stuff that matters in the end!