The Rebbe's guidance on peace between husband and wife:
Shalom bayis is paramount. Peace at home is always preferable to discord, and is worth yielding for; for example, in a disagreement over where to live, one spouse should give way with a full heart (IK 2713, 10822). Peace between husband and wife brings Hashem's blessings to the home (IK 7876, 10942).
Look at the good, and look at yourself first. Focus on the other's positive qualities rather than their faults, and examine oneself before criticizing one's spouse (IK 1276, 3897, 7510). Build real friendship and understanding (IK 2454). Every couple is different, and harmony takes ongoing effort, and sometimes compromise, from both sides (IK 7468, 9487, 10067, 10940).
Gentleness, never force. Change comes through gentle words and patience, not harsh words or pressure; avoiding anger matters for both peace and health (IK 732, 2653, 5557). A husband must take special care for his wife's honor, even in a disagreement (IK 8450). The wife's wisdom and positivity also play a great part in the peace of the home (IK 2732, 3042).
Getting help. When a couple can't resolve things themselves, consult a rav or a neutral, trusted mediator, together; even deep conflicts can be resolved with goodwill and gentle mediation (IK 3799, 4210, 5335, 6621, 9129, 10503). Relatives, even close family, should generally not intervene in a couple's disputes (IK 1489). A spouse's good friends can sometimes help gently (IK 2599).
Differences in religious practice: address them calmly and pleasantly, through dialogue and with a rav's guidance (IK 8097, 9703, 9704). But peace at home must never come at the expense of Torah observance itself (IK 5841).
Don't give up on the marriage. Even serious difficulties do not justify breaking up a Jewish home; seek guidance together and try every avenue first (IK 6727, 9990, 10503).
Spiritual steps: careful observance of taharas hamishpacha is often at the root of shalom bayis (IK 2938, 3097); check the mezuzos, give tzedakah, strengthen bitachon, and ask forgiveness where needed (IK 1141, 1294, 1661). Financial pressure also weighs on a marriage (IK 10219).
See also: נישואין – טהרת המשפחה.