If I was ever asked to give just one tip for making marriage great, it would be this:
Make God #1.
In your marriage, in your life, in your thoughts and actions.
When God gave the Ten Commandments, he began like this:
And God spoke all these words, saying,
“I am the Lord your God, who brought you out of the land of Egypt, out of the house of slavery.
“You shall have no other gods before me." Genesis 20:1-3
Having "no other gods" doesn't simply mean not worshiping false idols or the gods of other religions. It means not putting anything in one's life before God--and that includes one's spouse!
In the book of Matthew, a lawyer questioned Jesus and asked
“Teacher, which is the great commandment in the Law?” And he [Jesus] said to him, “You shall love the Lord your God with all your heart and with all your soul and with all your mind. This is the great and first commandment. And a second is like it: You shall love your neighbor as yourself. On these two commandments depend all the Law and the Prophets.” Matthew 22:36-40
What are we to do above everything? Love God with all our "heart, soul, and mind." In other words, with everything we have in us. And second, we are to love those around us as we love our spouses.
I don't want this to be all about what we do. Because ultimately, life isn't about us. Marriage isn't about us. It's about God. Life is a gift from him. Marriage is, in my humble opinion, one of the greatest gifts God's given us in this life. In marriage, God has given us a picture of the kind of sacrificial, selfless love he has for us. Of course, marriage doesn't always perfectly reflect that love--in fact, no marriage is ever perfect. That's because all marriages are between two sinful human beings. But God shows us grace, and blesses us despite our failures and shortcomings. He continues to pour out his love abundantly on us and our spouse, even when we don't deserve it. And God's love for us enables us to love one another.
The best thing any person can do for their marriage is to focus on God. Our spouse will fail us, and we will fail them. But God is always faithful, and we can trust that when we do fail, he will forgive us and renew us and our marriage.