well, I have had good examples to follow, grandparents and parents have both reached their golden weddings.
I think friendship is very important. If you marry someone who is your best friend, then you tend to treat them like a friend, love, respect, laughing together, shared interests and all the other things people have mentioned.
But I do think that deciding that you will stay married is also key. When we have a row, for me the option to leave is not an option I am prepared to take, so the only other option is to resolve it, which is where the compromise comes in.
I am not advocating staying in a bad relationship, but rather that I have chosen to make this one work. Mostly it is good, and not difficult, but we have had patches when it is dull or we are not getting on well, or we just don't like each other much. To me, that doesn't mean the marriage is over, it means it is time to touch base with each other and sort out what is getting between us. I suspect that many people give up at that point.
Mind you, I have a lovely man which helps - 15 years this year.