Good stuff here. Not much to add but here goes (in no particular order).
First, endeavour to marry the right person for the right reasons. This is difficult to quantify and is not one size fits all. However if you marry someone hoping they’ll change, you’ll likely be disappointed. Ironically, if you marry someone hoping they’ll stay the same, you will also likely be disappointed.
Attraction is important, but it’s not the end all. We all die old and ugly (if we’re lucky enough to make it there). That person who looks so good on your arm in their 20’s, 30’s and 40’s will start to go downhill in the looks department eventually. So will you. When that happens someone who is kind, respectful, funny, intelligent and helpful will be of much greater value than the person who used to be hot.
Treat your relationship like you would your bank account. Consistently put more into it than you want to take out or it’ll go broke. Also, when shit happens (and unfortunately it almost certainly will, probably more than once), you’ll have a reserve to carry you through. That said, throw away your scorecard. There will be times when you will feel like you’re putting more in than your spouse/SO. There will also be times when you actually ARE putting in more than your spouse/SO. This does not matter. Assuming your relationship is healthy, the shoe will eventually be on the other foot.
People value different things. If you express love through work/acts of service but your SO does so through cards and gifts, they may well not feel as though they are loved no matter how hard you bust your ass. This can be a real problem. Pay attention to how your SO expresses affection/gratitude/love and endeavour to respond in kind sometimes, even if it doesn’t come naturally.
Accept that in a dispute with a loved one, you can be happy or you can be right. You can very rarely be both. If you are trying to “win” a discussion, you have already lost. Never go to bed mad.
Invest in your relationship on an ongoing basis. This is easy in the beginning, but over the years weekends away together etc can begin to seem expensive and time consuming and even like a bit of a hassle. If you think they are too expensive/time consuming/a hassle, try a divorce. It’s my understanding that it’s much worse.
I will also second StrengthDawg’s grit and determination sentiment. Sometimes being married is more fun than others. If you expect “romance” to carry you through, you may be disappointed. I resolved when I stood up at my wedding and said the words that the only way I was leaving my marriage was toes first, in a bag. Period. Sometimes discipline and dogged perseverance trump passion in the moment. I’m not saying stay in a bad relationship (abusive, unfaithful, structurally incompatible etc), I’m just saying that sometimes even stuff that is worthwhile sucks for a bit. Gut up and embrace the suck.
Last, happy people make better partners. If you don’t like being you, odds are nobody else likes being with you. Follow your passions, own your crap and stay positive. Also, if respect is key in a relationship, as per John Gottman, PhD, you must respect yourself and have integrity, clear boundaries and expectations if you expect your SO to respect you.
Okay, I guess I had a few things to add after all. Sorry for text-wall.