I don't think there is any one particular "secret".
I'm a great believer in always treating one's partner as the most important and special person in one's life, and I treat mine with a lot of consideration and kindness. I would always say No to someone else, rather than inconvenience my partner.
Talk and listen a lot.
Try not to get into ruts and become boring.
Keep to high standards, and always make an effort for your partner.
Don't make your partner responsible for giving you all your happiness in life.
Don't get so intimate that there are no secrets, and no privacy - that's no way to keep passion alive. (No one really wants to see you on the loo, for instance, and partners don't need to know every little unpleasant detail of every small illness or ailment.)
I also believe that we are animals, and animals with a strong visual sense. I think so many people damage their relationships by thinking that their partners "ought" to love them for their beautiful souls, and it shouldn't matter if they turn into people who look very different from the attractive young people that their partners first fell in love with. So I think that staying slim, fit and attractive is much more important than people would like to believe. (Before anyone jumps on me, yes, I do believe that applies to men as well as to women.)
33 years here, and still going strong. I might even marry him one day!