Basics are what everyone says; honesty, respect for each other, responsible, trust, and good communication.
Some things that aren’t so obvious:
Checking with each other what your individual goals are, and supporting each other in pursuing them. Add in goals for you both as a couple. Often it’s things like financial goals, or something connected to finances. It can also be career, # of children, wanting to own something, level of education, hobby, the main point is if your relationship stifles you then getting out of the relationship starts to look attractive.
Communication. People really speak different languages. Being able to work through differences and being sure you really understand what your partner needs and wants is a real skill.
Don’t stay with someone who has red flags. Usually flags are there pretty early on, way before the wedding.
Be a good partner yourself.
Commitment. Staying together through hard times.
Relieving your spouse of the things they hate to do. I hate emptying the dishwasher, so my husband did it as his chore when I also worked full time. Now, in my current house (that we just had to sell :() he let me put in two dishwashers so I can avoid doing it longer.
Really liking spending time together. This includes having some things in common. This would include what you like to do to relax at the end of the day. Is it reading? Talking? TV? If one person likes to watch TV and laugh with their partner there, but the other person wants to read, that means possibly almost every evening the TV person will feel abandoned; or, the reading person, if they watch TV for the other person, will feel resentment. It’s not that it has to be a perfect match every day and night, but it can’t be so different that there is constant compromise or resentment or loneliness.