Gottman's book is definitely worth an in-depth read, so I don't feel like I'm giving away the store by repeating the titular 7 principles. (They're already revealed in the Wikipedia article anyway.)

1 Enhancing love maps
2 Nurturing fondness and admiration
3 Turning toward each other
4 Accepting influence
5 Solving solvable problems
6 Overcoming gridlock
7 Creating shared meaning