In the Bible we see examples of people starting out, looking faithful, but not carrying on in the way of righteousness for the long term. Perhaps "well-begun is half-done," but staying at it till done is faithfulness. So, though you may not cheat on your spouse during the early years of your marriage, if you do cheat later, you're not faithful. Faithfulness is till death do you part. And faithfulness is more than just sexual - it is also loyalty, self-giving, total commitment.
Faithfulness is cultivated in the tiny, mundane things of life.
When the first seeds of discontentment arise, "nip them in the bud!" Give thanks instead. Because, it is in the nurturing of the little evils that the big evils have their start. Replace arguing and grumbling with kind thoughts and deeds; replace laziness with hard work; replace discontentment with gratitude. So frequently marriages break up (or are a living hell) in later years because wrong thinking, attitudes, actions have been nurtured and allowed to fester and grow over many years till they are the dominating feature of the marriage. Again, they usually start small, almost unnoticeable.
Watch little things like your tone of voice, whether or not you're willing to do kind things for each other, whether or not you can read God's Word and pray together (which is actually not a little thing), how you talk about each other and/or tease each other.
These thoughts also carry into other areas, like your relationships to your siblings, friends, church elders, or other Christians.
Be careful of the little things, and the big things will fall into place.
Be honest with yourself; don't try to justify sin.
Be humble and correctable.
Be humble and not so eager to correct others.
Be vigilant and relying on the Holy Spirit.