honesty
what you mention is a very good recipe though. I had a gf like that once, all of her relationships have worked out because of this. No matter what happened during the day, she’d always make sure to get it sorted out by bedtime, and always end up having sex before sleep. The importance of this is that a fight is contained in one “off day”, rather than being drawn into the next day (and also registered in your dreams that night as a more permanent state).
No relationship can be constantly perfect, but if you have 25–28 good days in a month, that’s about as good as it gets.
My current gf is great in that she also tries to make up, even if it’s not her fault, and doesn’t keep a grudge. She always backs up, sometimes more than she needs to though. She’s been hurt in the past because of that. She values the relationship far more than her own individuality and self-esteem.
I have a different approach, though similar too: I may keep a grudge until the end of the day and spend some time alone or something, but the next day is always a fresh start, and no matter how it goes or how serious a fight may have been, I’ll always start anew every morning and act as if nothing’s happened. Usually we’ll sit down and try and resolve the issue quietly, but even if we can’t, I can isolate it as a minor exception to an otherwise great relationship and just accept that there was this one issue that wasn’t resolved and move on.