I don't think there's a single key to a long marriage, I believe that alot goes into making a marriage work but the most important thing is the willingness of both parties to actually want to make the marriage work.
For me I believe every marriage needs three things to work, namely;
1. Patience: This right here is one of the reasons why most marriages fail early, at least one partner isn't patient with the other and expects the other to adjust to their lifestyle immediately after they get married. Sometimes you need to be patient with your partner and let them slowly but surely come to terms with the fact that they're married and no longer a single entity.
Understanding: I can't see how any relationship would work if both partners aren't understanding of each other. You can't expect anyone to want to remain in a life long relationship with another person if they aren't understanding of who they are and what they pass through every day, it just won't work.
Communication: This is perhaps the most important thing every marriage needs, to be able to communicate with your partner and let him/her know what your feeling is crucial to the success of a marriage. If you can't communicate with each other properly then you're basically shackled by law to a wax dummy which will never be able to understand you.
I hope this helps.
To live with your partner forever, then supporting each other is the key to the happiness of your life. When your partner is down and falls, you don't leave it instead to strengthen and support it. Understand what he wants and needs.
The relationship between the two people is mutual, not unilateral. So fighting together and supporting each other will strengthen your relationship.
I'd say the number one thing is patience.
When talking with people I word that differently because I'm salty, but that's what it boils down to. You gotta be able to put up with a lot of crap, because nobody is as perfectly matched after "I do" as they were before it.