The four most important typographic choices you make in any document are point size, line spacing, line length, and font (see font recommendations), because those choices determine how the body text looks.
Point size should be 10–12 points in printed documents, 15-25 pixels on the web.
Line spacing should be 120–145% of the point size.
The average line length should be 45–90 characters (including spaces).
The easiest and most visible improvement you can make to your typography is to use a professional font, like those found in font recommendations.
Use curly quotation marks, not straight ones (see straight and curly quotes).
Put only one space between sentences.
Never use underlining, except perhaps for web links.
Use centered text sparingly.
Use bold or italic as little as possible.
All caps are fine for less than one line of text.
If you don’t have real small caps, don’t use them at all.
Use 5–12% extra letterspacing with all caps and small caps.
Kerning should always be turned on.
Use first-line indents that are one to four times the point size of the text, or use 4–10 points of space between paragraphs. But don’t use both.
Don’t confuse hyphens and dashes, and don’t use multiple hyphens as a dash.
Use ampersands sparingly, unless included in a proper name.
In a document longer than three pages, one exclamation point is plenty (see question marks and exclamation points).
Use proper trademark and copyright symbols—not alphabetic approximations.
Make ellipses using the proper character, not periods and spaces.
Make sure apostrophes point downward.
Make sure foot and inch marks are straight, not curly.