Although there are several very good suggestions, a lot of these recommendations might feed a codependency issue in someone who has issues prior to marriage.
The thing to remember about marriage is this: it is a legal, binding contract. It is not the blending of souls, a romantic "happily-ever-after" arrangement, or any of the other whimsical ideology that we've been taught to believe. A good, strong marriage requires individuals who are emotionally healthy who understand that the only person that they can control is themselves.
A spouse is not responsible for the state of mind of their partner. This doesn't mean that they cannot encourage and support - absolutely, support and encouragement are necessary for the other partner's self-esteem. However, the other partner must already have healthy "Self-isms" before plunging into the contract of marriage, or else their spouse will be beset with guilt, shame, hopelessness, helplessness, and fear.
There is NO simple means to create a "happy" marriage. "Happy" is a fleeting sentiment - Life is full of very UN-happy events and situations, and partners who are emotionally healthy to begin with will be able to weather those random Life's events in a healthy manner.