Okay, I don't count as "long married", but I do have 2 failed marriages (seriously awful stuff where I had to leave), oneof which lasted 13 years, and I'm in an incredibly happy relationship now - coming up on 3 years together. I know that doesn't sound like much, but I can pretty much guarantee we'll be together until one of us dies.
One thing I can point to as being drastically different in this relationship that was missing from the other relationships is how much we appreciate each other. We are constantly doing things for each other (and I don't mean buying things) and then we make sure we earnestly express our appreciation for those things, no matter how small.
I know it doens't sound like a big deal, but it makes a huge difference. Make sure your husband knows you've got his back no matter what, and do little things that he'll notice make his life a little better. It's contagious in a good relationship.
For instance, The Boyfriend is really bugged when the trash in the kitchen starts to get a little... tall. And he does seem to be the only one that ever empties it, so I took the trash out first thing this morning before he woke up. He worked late last night, so I made sure I left some deviled eggs in the fridge for him. I bring the paper in every morning so it's there when he gets up and doean't have to go out in the cold. Those are small things, but things he really appreciates.
He, on the other hand, washes my car without me ever asking, takes care of any travel plans (he's better at them) and volunteers to visit my mom with me. That just makes me love him even more, and I let him know that. He still opens car doors for me, and I still thank him every time. It just works.
I sent him an email once, "50 Reasons Why I Love You", and he said it took everything he had to keep from forwarding it to everyone he knew. I also wrote a fairy tale version of how we met, and he still has it in the drawer next to the bed years later. It makes him feel special, and I think that's one of the most important factors in keeping a love affair strong.