We were together for 8 years before we got married. "Rules" were already established well before the wedding day. Hahaha
I think that is an unspoken rule of ours. I would say for us that was just a given. Plus how bad would things have to be to even utter those words. Shit would HAVE to already be pretty bad before that was even mentioned anyways.
We also don't spend over $100 on something unexpected without discussing it with the other. This has saved each of us from making impulse purchases that we would have regretted. He doens't just go out and buy a new computer, and I don't just go out and sign up for things from door to door salesmen without being able to research them first. True story... I was so glad I didn't just buy it, because I can buy them 1/3 of the price online. Items that are typically over that amount are budgeted for. I.e. groceries, new clothes for kids, birthday and Christmas gifts
We also don't cultivate friendships with members of the opposite sex away from each other. Meaning I don't go hang out with one of our guy friends at a coffee shop. This helps us stay "affair proof".
We have sex every week... meaning at least 1 time in a 7 day period, unless there's a serious medical reason not to. Usually it's more often than that, but that's my promise to him. Arguments, schedules, and other bothersome things just have to be worked through and not left to fester.
Mutual Respect is the biggest. Communication,no matter how hard a topic. Choose your words carefully in an argument-you can't take back things said in anger,so you better mean what you say when you say them.