Acceptance
It's critical to take the bad with the good. The key to this is understanding that often times the things that we love about people are also the things that drive us crazy about them. My wife tells a story about how impressed she was at how hard of a worker I was when she first met me. Then years later she told another story about me never being home and always working....(more about this in a min)
I think a lot of stress comes from trying to change your partner or wishing they would change. But the reality is that you must resolve to love all of them, flaws and all. Once you feel this acceptance from someone, it creates a safety in the relationship that feels amazing. "She knows me really...and still accepts me"
Perseverance
The going WILL get tough. It's almost always easier to abandon ship than it is to repair the one you're on. The thing I've learned though in 17 years is that nothing ever lasts-good or bad times. When you go through a rough spot, no one can tell you how long to endure but I can tell you that from my years of marriage, adventure racing, ultra-marathoning, crossfitting, etc...that you are ALWAYS capable of persevering a little more than you think. So when you are struggling in a relationship just try to make it another day, hour, minute, whatever you have to do to stay in the fight.
Over the years when my relationship was at low points this understanding became invaluable. It also is another example of the principle of your weaknesses and strengths being 2 sides of the same coin. I may have been stubborn in an argument over the years, but I was equally tenacious in hanging in there and not giving up on the relationship.
Self-awareness
Self awareness may be the most elusive but also the most important piece to not only marital but also individual happiness. When you know yourself well, you become aware of the triggers that can cause problems. We all have them. They usually stem from a feeling of not being accepted (see above...) or something that says you are not good enough. When we really know ourselves, we can spot these triggers for what they are, not really happening in the present, but something being brought out of the past from our present situation.
Self-awareness allows you to communicate more out of love. When you realize that certain triggers and fears are not really what's going on in the present situation it lets you slow everything down and give your partner what they need. When I realized that my value wasn't tied to my work...I started to be home more.
So for what its worth, these are 3 things that jumped out at me when my buddy told me about his engagement. The 1st thing I said is, "Congrats. Now the real work begins". And it's true. A fulfilling and rewarding relationship takes work. But oh is it ever worth it.