Form the possessive singular of nouns by adding 's. Follow this rule whatever the final consonant (Charles's friend, Burns's poems, the witch's malice). Exceptions are the possessives of ancient proper names ending in -es  and -is,  the possessive of Jesus',  and such forms as for conscience' sake, for righteousness' sake. 

The pronominal possessives hers, its, theirs, yours,   and ours  have no apostrophe. Indefinite pronouns, however, use the apostrophe to show possession (one's rights, somebody else's umbrella) (1).

Omit needless words. Vigorous writing is concise. A sentence should contain no unnecessary words, a paragraph no unnecessary sentences, for the same reason that a drawing should have no unnecessary lines and a machine no unnecessary parts. This requires not that the writer make all sentences short or avoid all detail and treat subjects only in outline, but that every word tell (23).

Numerals. Do not spell out dates or other serial numbers. Write them in figures or in Roman notation, as appropriate.

August 9, 1988
Rule 3
Part XII
352d Infantry

Exception: When they occur in dialogue, most dates and numbers are spelled out: "I arrived home on August ninth." "In the year 1990, I turned twenty-one." "Read Chapter Twelve." (35).

Effect. As a noun, means "result"; as a verb, means "to bring about," "to accomplish" (not to be confused with affect,  which means "to influence").

As a noun, often loosely used in perfunctory writing about fashions, music, painting, and other arts: "a South-western effect"; "effects in pale green"; "very delicate effects"; "subtle effects"; "a charming effect was produced." The writer who has a definite meaning to express will not take refuge in such vagueness (45).

Write in a way that comes naturally. Write in a way that comes easily and naturally to you, using words and phrases that come readily to hand. But do not assume that because you have acted naturally your product is without flaw.

The use of language begins with imitation. The infant imitates the sounds made by its parents; the child imitates first the spoken languauge, then the stuff of books. The imitative life continues long after the writer is secure in the language, for it is almost impossible to avoid imitating what one admires. Never imitate consciously, but do not worry about being an imitator; take pains instead to admire what is good. Then when you write in a way that comes naturally, you will echo the halloos that bear repeating (70).