The ones who achieve long-term happiness (or fulfillment), what are they doing differently?
platitudes about communication will not suffice here
Someone said that no size fits all, and every relationship is different.
However, both my parents and my in-laws celebrated 35 and 30 years, respectively. They say a few things have helped them over the years:
1. Communication. (It's sad when a cliche is true, isn't it?)
2. Regular sex. (2-3 times/week)
3. Cooperative decision making. (ie., no one person rules the roost)
4. Regular time together. (Date night once a week)
5. Physical/emotional/mental fidelity. (like Diemen said, it's not difficult if you want to be)
6. The willingness to sacrifice for the sake of the other person -- on both sides.
7. Keeping work in its appropriate place. (don't be consumed by a job or career)
8. Remembering that the kids are guests in your home -- guests who will someday leave.
9. Having a network of support -- church, close friends, etc. -- people who can tell you when you're wrong.
I think the reality is that the biggest thing to making a relationship/marriage work is choosing the right person. As has been said before, sexual compatability is not the same thing as relational compatability, but an awful lot of decisions are made based on chemistry, not compatability. Or they get married, and one person -- or both -- get focused on other things ("hey, I married you, didn't I?"), and are then surprised when they wake up 2, 3, 5, 10 years into a marriage and say, "I don't know this person anymore." The wedding day is just the start of everything.
It leaves you, baby, if you don't care for it.