Avoid complacency like the plague.
Perhaps more than any other factor, complacency destroys romantic relationships. Imago Relationships International warns those in long-term relationships to fuel the fire of each others' passion. Otherwise, the marriage is at risk for loss of intimacy, with infidelity not far behind.
Imago claims it is not boredom in the bedroom that causes couples to stray. Complacency and ignoring each others' new found passions, pursuits and creative endeavors opens the door for unhappiness in marriages.
Be proactive with your love.
Don't wait until you find an illicit email that exposes your cheating partner. Take the initiative to keep the relationship healthy and prevent problems from developing.
Have you considered taking a couples' communication skills course, just because it is a good thing to do? Do you hold regular planning meetings with each other to make sure your relationship priorities are appropriate? Do you take thoughtful action to let your lover know she is loved before she has a chance to question it?
Wake up, couples! Procreative love is a fire that can warm your life, or burn it down.