Good communication
I agree this is the cornerstone, because if you can do this you can meet each other's needs.
I genuinely believe marriage is about having your own needs met, and meeting the other persons needs and the commitment lies in always wanting to meet the other persons needs where you can.
The hidden problem can be that people bring in their own baggage and agendas into new relationships and sometimes you can both be trying your hardest, doing everything right (as you see it) and still messing it up because you don't actually KNOW what your own needs ARE.
Sometimes what you think is right is not always truly right and good people with the best intentions can end up divorced for exactly this reason.
To make sure communication is really open, I'd advise any couple to have a monthly grievance session where each one can talk openly about where their head is at.
This might seem silly, but when I began my relationship with my SO, he talked about communication being paramount and while he thought he was communicating with me -he wasn't. He was doing so selectively, and in the end he resented me for not meeting his needs - although he had never been truthful about what those were. We are now in counselling for that and very nearly ended up separated for good because he was bottling little things up.
An example of how "with the best intentions" and all the right ideas, it can still very easily be lost -especially when hard times come, and they always do.