The problem with all the following is it relies on being married to someone lovely, who genuinely has your best interests at heart and wants the best for you and for the family. If s/he doesn't it's a recipe for doormat status instead. But:
Be kind. Don't speak contemptuously to one another. We do fight but they're clean fights, for want of a better word. I cringe when I hear how some people argue, and research actually shows contemptuous comments are the best indicators of marriages that will fail. You can't be emotionally vulnerable and intimate with someone who makes you feel uneasy and judged.
Don't play games. Be honest, talk, share stuff. Even if it's an anecdote or bit of gossip, or a DVD or new book. Ensure you don't end up flatmates who run a daycare.
Express appreciation for all the other does for you personally, and for the family as a whole. Say what a great parent/cook/book-keeper they are. Be appreciative of their friendship, their love and their body. 
Try to resolve disputes through affectionate teasing. It's a lot easier to hear the other person via a loving send up than a critical rant.
Have sex regularly if not too tired/post-natal/ill. Sex is a habit and one you can fall out of. But don't ever pressure someone not into it - DH never does and nor do I, even through droughts, which IMO (plus some willing on both sides) is why ours never lasted.
Don't be lazy with one another, in practical and emotional terms. Mention that you adore them, that they're special and wonderful, and you're glad they're with you.
Stay faithful. This is not down to chance. It has to be an ongoing decision to boundary your marriage as the most significant relationship you have.
When you fuck up, don't make excuses. Apologise and then make amends.
Poem by Ogden Nash: "To keep your marriage brimming, With love in the loving cup, Whenever you're wrong, admit it; Whenever you're right, shut up."
I think "be excellent to each other" covers the bases.
But luck plays a huge role as well.