"Don't let your kids become the only thing you can talk about with your spouse. If that's the only topic of conversation, that's a big red flag.
Fighting is important. My mom tells me all the time that the opposite of love isn't hate, it's indifference. Not sure that's entirely true but I think she makes a good point. You don't want to bicker constantly but if you become indifferent, that's a bad sign too.
It's the little things that go a long way when you're married for years. Love notes, flowers for no reason, thoughtful little gifts. These things all add up. Just because you're married doesn't mean you can stop trying.
Don't live separate lives. My mom was always so busy with us and my dad was working a ton. Even though we ate dinner as a family every night, sometimes it felt like my parents were living different lives despite living together. I think it's important for kids to see their parents doing things just the two of them (date nights, activities, etc).
TL;DR: Marriage takes work. A LOT of work. Even after 23 years (the length of my parents' marriage), things can fall apart."