Disclaimer: these are things that keep my marriage strong. Your marriage is between you and your spouse and you need to do things that work for you.
Commitment, which unfortunately includes putting up with stuff you might not put up with from someone you weren't married to. When old married couples say, "we fixed things when they broke rather than throw them away," that means they put up with stuff long enough to find a compromise or resolution.
Communication. No one can read minds. Reading between the lines, picking up on signals, taking hints, or "if you loved me, I wouldn't have to tell you," all lead to disaster. Just blurt shit out. "I don't like when you..." "I need ... from you." "I want to ..."
Honesty. If a marriage is going to last, you can't lie a lot. For one thing, you'll trip up and get caught. But for another, you'll resent feeling like you have to lie. They're not your parent, they're your partner. If the consequences of doing something aren't worth it, just don't do it.
Politeness. Don't be rude just because you're in your own house. It's a shared home and it's theirs too. Say please and thank you. Try to minimize burps and farts. Warn them when you've destroyed the bathroom (and light a candle).
Compliment them. Their outfit, the dinner they made, their new haircut. And if there's nothing particular right now to compliment, just tell them how lucky you are to have them. If they're struggling, compliment them more. For one thing, it's nice to hear compliments. But for another, positive motivation works much better than negative motivation. "I noticed you cleaned the kitchen, it looks really good," will get the kitchen cleaned more than, "Could you clean the kitchen more, it's filthy."
Trust. Make a conscious decision to trust the person. Most suspicions come from personal insecurities rather than actual suspicious behavior. Let him have female friends. Let her have male friends. If someone else flirts with your partner, trust your partner. Or if you must say something to the flirter, try, "I saw you talking to my spouse. I can't blame you for flirting -- believe me I see why you would -- but there's no point. (S)he's married and we don't have an open marriage." And set realistic boundaries. In a lifelong marriage they WILL find someone else attractive, probably lots of people. It's unreasonable to expect them not to. It is reasonable to expect them not to act on it, and not to tell you about it, and to try to be discreet. But if you have a beautiful friend he's going to know they're beautiful. You can either hassle your partner and drive them away or decide to trust them. If they're going to cheat, they're going to cheat. Being suspicious and jealous and forbidding friendships has never kept someone from cheating. But it has pushed people to it.
