Open communication and mutual trust.

Persistence and respect. You're bound to have rough patches, but you have to decide together that it won't break you, that you're still a team. Respect is important because it colors your interactions. Love is wonderful, but if you think "What a drunk" or "Mama's boy" when you look at him/her, you should be with someone else, because you will at some point cease to treat them with the common decency everyone deserves. No one likes to see that cranky couple coming, especially if they have children.

Respect and communication, no to mention being emotionally healthy or on the same emotional level.

Respect if you don't have any for each other it' just not going to work

Great communication, absolute trust, and loyalty.