1.) Stop communicating.
Ephesians 5:23-30 says,
"For the husband is the head of the wife, as Christ also is the head of the church, He Himself being the Savior of the body....Husbands, love your wives, just as Christ also loved the church and gave Himself up for her...So husbands ought also to love their own wives as their own bodies. He who loves his own wife loves himself; for no one ever hated his own flesh, but nourishes and cherishes it, just as Christ also does the church, because we are members of His body."
In life, our bodies constantly communicate with our heads, and vice versa. What happens if suddenly that communication stops? Usually paralysis, and if it's bad enough, death. Communication both ways is needed for the the organism to survive, and in marriage we are considered "one flesh" and thus one organism. If you want to kill any relationship, stop communicating. It'll be dead before you know it.
2.) Confide in a friend of the opposite sex.
What routinely kills marriages is one partner developing strong emotional bonds with someone of the opposite sex. This is true of any romantic relationship. If you need emotional support, go to your spouse. Your spouse must be your best friend. 1 Cor 13:4 reminds us that, "love does not envy," and if you're making your spouse jealous for the bond he/she wants to have with you, you're introducing something which is certainly not love.
If you let someone else take the place of your partner emotionally, sexually, or physically, you're making a choice to tear down everything that you've built.
3.) Demean or insult your spouse.
Eph 5:29 points out that in a marriage, "no one ever hated his own flesh, but nourishes and cherishes it, just as Christ also does the church." If you are belittling your spouse instead of encouraging him/her, there's no nourishment going on. The relationship will grow as well as a plant watered with paint thinner, because insults and belittling statements cause envy and are utterly toxic.
It's worth noting that comparing your spouse or romantic interest negatively to someone else is the absolute most deadly thing to do. "Well Jerry is more manly," or, "Jessica's a lot more feminine" shows that not only do you not appreciate the one you claim to love, but that you're thinking about someone else, too...and viewing them as a more desirable mate.
4.) Stop having sex.
Sex is a great thing which God has provided, and is an intimate thing between only you and your spouse. It is a bond that has been committed to with words, and, at a physiological level, even with chemicals. It you stop seeing it as important, your relationship will almost certainly head downhill, unless there is mutual apathy, in which case it's already in trouble. Look at the importance that God places on it in 1 Cor 7:3-5,
"The husband should fulfill his marital duty to his wife, and likewise the wife to her husband. The wife's body does not belong to her alone but also to her husband. In the same way, the husband's body does not belong to him alone but also to his wife. Do not deprive each other except by mutual consent and for a time, so that you may devote yourselves to prayer. Then come together again so that Satan will not tempt you because of your lack of self-control."
If you stop making love, you give Satan another avenue to tempt and try to destroy a marriage.
5.) Start keeping secrets.
I once saw a post that discussed relationships which were budding, and it said:
“The Detective Rule”. "The idea is simple: if you find yourself playing detective with someone, you remove them from your life immediately. Remember your Constant? Do you play detective with them? Do you cyberstalk their Facebook page and question their every intention? No, of course not. So you know the common denominator is external."
God speaks in Job about trust, and there are two types which play out. In Job 11:18, there is trust that is solid, "“Then you would trust, because there is hope; And you would look around and rest securely." Notice the confidence and ability to rest well and be at peace. On the other hand, if our spouse keeps secrets, we start to have "confidence that is fragile, and trust that is a spider's web." (Job 8:14)
Don't keep secrets. You'll both be able to sleep easily at night.
6.) Spend lots of time around people who don't like your spouse or loved one.
I have seen this one happen a LOT. Every time I've seen it, it's been the mother or father (or both) who hate who their child has married or is in love with. This is a recipe for disaster because, as Matthew 6:24 notes, "No one can serve two masters, for either he will hate the one and love the other, or he will be devoted to the one and despise the other."
Suddenly the spouse will be trying to maintain the relationship with the friends or family who dislike who he/she has married, and in doing so, they have to subtly ally themselves with these individuals.
If that happens, we need to remember that, "For this reason a man shall leave his father and his mother, and be joined to his wife; and they shall become one flesh." (Gen 2:24) A new family has been made, and bitterness from an old family cannot be present. The husband and wife must be one flesh, and must both refuse to allow an emotional war to take place.
7.) Quit trying.
"Courage doesn't always roar. Sometimes courage is the quiet voice at the end of the day saying 'I will try again tomorrow.'" —Mary Radmacher
If your goal is success, you must keep striving toward it. If neither partner gives up, success will result, but if just one partner gives up, destruction is guaranteed. "And let us not grow weary of doing good, for in due season we will reap, if we do not give up...Not by might, nor by power, but by my Spirit, says the LORD of hosts." ( Gal 6:9 & Zech 4:6)
"For the entire Law is fulfilled in in this one word: "You shall love your neighbor as yourself."—Gal 5:14