The close of a less intimate letter, like taking leave of a visitor in your drawing-room, is necessarily more ceremonious. And the "ceremonious close" presents to most people the greatest difficulty in letter-writing.
It is really quite simple, if you realize that the aim of the closing paragraph is merely to bring in a personal hyphen between the person writing and the person written to.
"The mountains were beautiful at sunset." It is a bad closing sentence because "the mountains" have nothing personal to either of you. But if you can add "—they reminded me of the time we were in Colorado together," or "—how different from our wide prairies at home," you have crossed a bridge, as it were.
Or:
"We have had a wonderful trip, but I do miss you all at home, and long to hear from you soon again."
Or (from one at home):
"Your closed house makes me very lonely to pass. I do hope you are coming back soon."
Sometimes an ending falls naturally into a sentence that ends with your signature. "If I could look up now and see you coming into the room, there would be no happier woman in the whole State than
Your devoted mother."
Letters No One Cares To Read