Adding to his comments, Dr. Frederick noted that though many of the couples read self-help books, those that were having passionate sex were the ones actually trying out the spicy ideas in said books. More specific habits of those with happy sex lives include: cuddling, deep kissing, laughing together during sex (though probably with each other and not at each other), saying "I love you" during sex, sending flirty text messages before sex, and trying out new toys, fantasies, positions, and lingerie. Basically, the message is, people with continually wonderful sex lives are continually adventurous, fun, and open people.
So, there you go. If you want have fantastic sex for the long term, try butt stuff. And obviously other stuff, too.