"Kol asher tomar eilecha Sarah, shema b'kolah" - "Avraham, whatever Sarah tells you, listen to her voice." Anyone who is married can probably attest to the fact that this command was probably was as difficult a test as any of the others Avraham faced : ) But in seriousness, the pasuk is actually a key to marital harmony. The Radomsker explains, "Shema b'kolah" may be read derech derush to refer to "Shema Yisrael, Hashem Elokeinu Hashem Echad", the ideal of mesirus nefesh, which Sarah personified in everything she said and did. Sarah's only concern was the welfare of others, not her own ego. Imagine if every time your spouse spoke to you the words you heard were, "Shema Yisrael...", i.e. I am only saying this l'shem shamayim for our improvement. How could you not listen?
On the same topic of shalom bayis, it is noteworthy that when the three people/angels come to Avraham's house, they make a point of asking where Sarah is. Rashi comments that this was done "l'chabivah al ba'alah", to make Avraham cherish Sarah, as he would be reminded of her tzniyus. R' Chaim Shmuelevitz points out that Avraham and Sarahat ages 99 and 90 respectively must have been married a good many years before this moment. Yet, even after so many years of marriage, there was still room for Avraham to appreciate his wife even more!
My wife added that perhaps davka because Avraham and Sarah had been together for so long this reminder was necessary. When a couple is young, every experience they share together is new and precious. The first time taking out the garbage, doing the dishes, cleaning for Shabbos - isn't it great to be married and in your own house! But after 80 years of taking out the garbage, doing the dishes, helping clean for Shabbos - the bloom has long since faded from the rose. But even after 80 and 90 years, a "l'chabiva al ba'alah" is still possible.