Not to sound too pat, but I wrote a Hub describing the things that have helped my wife and I to enjoy our 42yr marriage (and counting..). The end summary reads;
So, what’s the secret to a long and fulfilling marriage?
1: Same as it ever was: Love. Respect. Honour. (Like it says on the cue card..)
2: Remember, you get married to be with each other; not to become each other.
3: If you're looking for a long term relationship where the person opposite you is always in agreement; don’t get married. Get a mirror.
To us, it's about being true to yourself, and not just recognising each others differences, but actively enjoying those differences. Love the one you're with, don't try and mould them into some other ideal.
Of course, if you're not in love, don't even begin the journey.