A new survey of people in long time relationships to the world:
Love is an action, not an emotion. There will be times when the warm fuzzies are ubiquitous, and times when they aren't. Show love and demonstrate it, even when you don't “feel” it. Life with two humans is inherently complicated, and the feelings are going to eventually come back, if both parties keep working at it. Don't throw away a good marriage because the feelings faded for a time.
Communicate. Talk about stuff. 9 times out of 10 they weren't trying to hurt or annoy you and they didn't do it on purpose.
There's no such thing as winning a fight with your significant other.
Recognize that your partner is not you. They have a separate brain and self-awareness and perception of the world. They have a whole set of different life experiences, and consequently, a whole different way of seeing, reacting, and understanding. No, they aren't going to do something exactly the way you would do it. No, their first instinct might not be your first instinct. No, they are not you. And you know what? That's why you're with them.
Having a similar type of humor and laughing frequently is a MUST.
Have a shared sense of purpose. Talk about why you live. Why you choose to work. Why you choose to marry. Talk through these big ideas often. Go back to them.
Learn to compromise. Marriage makes both people different. You will lose a bit of who you are at the edges, but if you do it right, you'll be a better person in the end. Marriage isn't about holding on to your essential self. The romantics got this one wrong.
Sex is great but it's over emphasized. Have lots of fun sex as a married couple but never think it's why you are married. When it gets boring and utilitarian, don't stress. It'll probably get better eventually. Work it out. Sex is part of the long, loving conversation of marriage. It's not the professional wrestling version of sex you see in porn.
Don't cheat. Don't drink a ton of booze. Don't do a ton of drugs. Pay your bills on time. Life is for grown ups. Grow the hell up and get to work. It's worth it when you're old and loved and you managed to help some people out along the way.
Build your partner up and help them be the best version of themselves. Make sure you are with someone who will do the same.
Do things for your SO without an expectation to be repaid. Learn to love to have them happy. From my experience, they will do the same in return.
The big key to learning how to talk kind and be kind? Root out every way of expressing yourself that could fall into the basic category known as “passive-agressiveness.” Read up on how to recognize (in yourself and in others) biting, bitter, and resentful undertones, and then resolve to never, ever stoop to that level of immaturity and lack of clarity. Learn to express what you really mean. And equally, when you detect resentfulness in your partner, stop the conversation there and then, and ask, “I'm hearing resentment. Are you feeling resentment?” This teaches you both how to improve your communication skills. Never left a conversation continue where things aren't being said. Break the chain.