By continuing to "grow together" and wanting the same things for the marriage. Never stop being the person they fell in love with.
People often say marriage requires a lot of "hard work". However if you've chosen "the right mate for yourself" the so called "work" is actually (maintenance) or could even be seen as a "labor of love".
Imagine a person with a garden. They're constantly doing something to tend to it whether it's digging in dirt, planting new seeds, watering and using fertilizer, pulling up weeds, or building a fence to keep out pests. For the person (who does not want a garden) these activities could be seen as "hard work".
However for the person who actually (wants a garden) and enjoys seeing the finished results of their time and efforts are likely to view this "work" as a "labor of love."
Work in my opinion is defined as any activity I'd rather not be doing.
No one has to "get married". However if it is what (you want) or have (chosen) for yourself then the mature thing to do is to nurture and maintain it as best as you can.
Marriage is much like a garden, it will die from neglect.