If a number begins a sentence, write it out. Some style books make an exception for years, and I support this since we're used to seeing years written as numbers.
Incorrect: 12 persons compromise a jury. Nineteen fourteen was a cold and bitter year.
Correct: Twelve persons compromise a jury. 1914 was a cold and bitter year.
Incorrect: Twelve hundred and forty-four dollars sixty cents.
Correct: Twelve hundred forty-four dollars and sixty cents.
Hyphenate compound numbers and fractions between 21 and 99 (twenty-one and ninety-nine).
Incorrect: Twenty one, eighty nine, one-hundred-forty-three. One eighth.
Correct: Twenty-one, eighty-nine, one hundred forty-three. One-eighth.
Express large numbers as simply as possible.
Incorrect: One thousand two hundred and thirty-three.
Correct: Twelve hundred thirty-three.
Use commas to separate large numbers into groups of threes. This is not law, but it makes life easy.
Incorrect: 12435761 (This is not wrong, but quick, what's this number?)
To save confusion between 12:00 AM and 12:00 PM, use noon and midnight instead.
Incorrect: Meet us at 12:00 AM. (The writer may have meant noon, but was confused.)
Correct: Meet us at noon. (Regardless whether the writer knows his AM from PM, he knows the difference between noon and midnight).
Bible verses should not be written out.
Incorrect: John two : fifteen
Correct: John 2:15
Do not use apostrophes for numbers pertaining to decades or centuries.
Incorrect: I was born in the 90's. Herbert studies music of the 1800's. I have 100's of books.
Correct: I was born in the 90s (or nineties). Herbert studies music of the 1800s. I have hundreds of books.
There are multiple proper ways to write dates, and the following are examples of a few. I particularly like the first, because it separates the numerals and lacks commas, making it the fastest to write, read, and understand.
19 September 2015
September 19, 2015
Saturday, September 19, 2015