Rule #1: Communicate.
Rule #2: No, seriously. Communicate constantly.
Rule #3: Dude! I mean it! Freaking tell each other everything!
Rule #4: Always deal with your partner as a partner, and someone whose situation you are actively seeking to optimize, rather than as an adversary. Make their happiness as much of a goal as your own. You can have a disagreement with someone and still be philosophically "on their side"!
Rule #4.5: Corollary- In dealing with your partner, assume that they're doing the same thing, and working toward your common good. In CorpSpeak this is referred to as "assuming positive intent", and I think that it's far more applicable to intimate relationships than it is to work-related ones. If for some reason you can't open yourself up enough to trust your partner to have your best interests at heart even during a disagreement then it may be time to seriously re-evaluate the relationship as a whole.
Rule #5: When you're wrong, whether it's on the entire issue or simply one small point, admit it! Trying to defend an untenable position is frustrating and pointless! Conversely, by abandoning a point where you're wrong, apologizing sincerely, and making appropriate amends, you get to be no longer wrong!
When you're on the same side, "winning" an argument doesn't come from simply getting "the other side" to concede. It comes from coming to a mutual agreement about the situation and the appropriate course of action, and moving forward together.