What Happens ...

I know I have mentioned many times that Husband and I have been together for 18 years and married for 9(next week:), but I still want to know is the is a secret to having a long and happy life together and if so what is it? And what I really want to know is what happens to your relationship after your kids grow-up and move out? Does your relationship just grow as your kids do? So, I am just wondering.