If the Letter is to be in answer to another, begin by getting out that other letter and reading it through, in order to refresh your memory, as to what it is you have to answer, and as to your correspondent’s present address (otherwise you will be sending your letter to his regular address in London, though he has been careful in writing to give you his Torquay address in full).  Next, Address and Stamp the Envelope. “What! Before writing the Letter?” Most certainly.

And I’ll tell you what will happen if you don’t. You will go on writing till the last moment, and just in the middle of the last sentence, you will become aware that ‘time’s up!’ Then comes the hurried wind-up—the wildly-scrawled signature—the hastily-fastened envelope, which comes open in the post—the address, a mere hieroglyphic—the horrible discovery that you’ve forgotten to replenish your Stamp-Case—the frantic appeal, to every one in the house, to lend you a Stamp—the headlong rush to the Post Office, arriving, hot and gasping, just after the box has closed—and finally, a week afterwards, the return of the Letter, from the Dead-Letter Office, marked “address illegible”!

Next, put your own address, in full, at the top of the note-sheet. It is an aggravating thing——I speak from bitter experience——when a friend, staying at some new address, heads his letter “Dover,” simply, assuming that you can get the rest of the address from his previous letter, which perhaps you have destroyed.

Next, put the date in full. It is another aggravating thing, when you wish, years afterwards, to arrange a series of letters, to find them dated “Feb. 17”, “Aug. 2”, without any year to guide you as to which comes first. And never, never, dear Madam (N.B. this remark is addressed to ladies only: no man would ever do such a thing), put “Wednesday”, simply, as the date!

“That way madness lies.”

from Eight or Nine wise Words About Letter-Writing