1. YOU NEVER COMMUNICATE: Talk. Check in with each other regularly. Be sure to verbalize what's wrong, but also what's right with the relationship. Discuss what to do to make you both happy. If you're having trouble communicating, get professional help from a marriage counselor or therapist.
2. YOU ARGUE ALL THE TIME: No one wants to fight all the time. It just takes all the fun and joy out of your relationship. It also can make you both bitter and angry. Pick your battles carefully. There are ways to disagree, so that you are respectful and you both hear each other out. Also, you can learn to agree to disagree on certain topics.
3. YOU ARE HIDING THINGS FROM ONE ANOTHER: Honesty and openness is what your marriage is built on. When you start hiding things like text messages, friendships with the opposite sex, etc ... you start setting relationship up for HUGE trouble.
4. YOU CONSTANTLY SPEND MORE THAN YOU MAKE: Making money mistakes - failing to get out of debt, spending what you don't have, or making poor investments - is cause for more arguing and sometimes a break down of communication (the other killers on this list).
5. YOU STOP PURSUING ONE ANOTHER: By far, one of the easiest ways for couples to kill their marriage is when they stop pursuing one another. Your goal everyday should be to work just as hard to keep your spouse as you did to win him/her in the first place. Never get comfortable in your marriage by just thinking, "I don't have to work at it ... everything will be fine on its own."
6. YOU'VE STOPPED HAVING SEX: A marriage without sex can definitely wither and die, especially if one of the people in the marriage is unhappy about this. Sex is what separates the marriage relationship from other relationships you have. Don't forget ... never ever give up on your marriage. Work hard to fix any area that may be broken.