Seek help early.
The average couple waits 6 years before seeking help for relationship problems. (And keep in mind, half of all marriages that end do so in the first 7 years). This means the average couple lives with unhappiness for far too long.
Edit yourself.
The happiest couples avoid saying every critical thought when discussing touchy topics.

Soften your "start up."
Arguments often "start up" because one partner escalates the conflict by making a critical or contemptuous remark. Bringing up problems gently and without blame works much better.

Accept influence from your partner.
In studying heterosexual marriages, we found that a relationship succeeds to the extent that the husband can accept influence from his wife. For instance, a woman says, "Do you have to work Thursday night? My mother is coming that weekend, and I need your help getting ready." Her husband then replies, "My plans are set, and I'm not changing them." As you might guess, this guy is in a shaky marriage. A husband's ability to be influenced by his wife (rather than vice-versa) is crucial - because research shows that women are already well practiced at accepting influence from men.  A true partnership only occurs when a husband can do the same thing.

Have high standards.
Happy couples have high standards for each other. The most successful couples are those who, even as newlyweds, refused to accept hurtful behavior from one another. Low levels of tolerance for bad behavior in the beginning of a relationship equals a happier couple down the road.

Learn to repair and exit the argument.
Happy couples have learned how to exit an argument, or how to repair the situation before an argument gets completely out of control. Examples of repair attempts: using humor; stroking your partner with a caring remark ("I understand that this is hard for you"); making it clear you're on common ground ("We'll tackle this problem together"); backing down (in marriage, as in the martial art Aikido, you often have to yield to win); and, in general, offering signs of appreciation for your partner and his or her feelings along the way. If an argument gets too heated, take a 20-minute break, and agree to approach the topic again when you are both calm.

Focus on the bright side.
In a happy marriage, while discussing problems, couples make at least five times as many positive statements to and about each other and their relationship as negative ones. For example, "We laugh a lot" as opposed to "We never have any fun." A good marriage must have a rich climate of positivity. Make regular deposits to your emotional bank accounts!

--- Courtesy of the Gottman Relationship Institute gottman.com