Years ago I went on a trip to Hawaii by myself to write and meet new people. Along the way I noticed quite a few couples over 70 who gave off the impression they had been together for a long time. I went up to each couple and asked them if they would answer a few questions. I wanted to know how long there were married, what kept them together, and how they felt about each over having spent so many years together.
Across the board, they stated almost the same things. They said it was critical to say thank you and I'm sorry when appropriate and that even those small polite statements helped to glue them back together when conflicts arose.
They said it was always important to go to bed having cleared up matters for the day. Even with children and when things were overwhelming, they made sure to kiss or hug before climbing into bed. Sometimes they said they resolved to discuss or finish a matter at a certain date, such as, on Friday night at 8 or Saturday at 12. That helped them put a matter to rest so they didn't carry it around and fume over it.
One couple insisted that it was imperative to the health of a marriage to have at least one hobby or activity the couple could do together, be it golf, bowling, drawing, boating, etc.
The one thing that each stated emphatically, and I interviewed maybe 8 couples, was that they made sure to never assume what their partner was feeling, wanted, or needed. The couples insisted that thinking you knew the mind and feelings of the other caused more problems because it made the other person feel ignored and unimportant.