The four most important typographic considerations for body text 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).
Use bold or italic as little as possible, and not together.
Never underline, except perhaps for web links.
All caps are fine for less than one line of text.
Use centered text sparingly.
Put only one space between sentences.
If you don’t have real small caps, don’t use them at all.
Kerning should always be turned on.
Don’t confuse hyphens and dashes, and don’t use multiple hyphens as a dash.
Use ampersands sparingly, unless included in a proper name.
Use proper trademark and copyright symbols—not alphabetic approximations.
In a document longer than three pages, one exclamation point is plenty (see question marks and exclamation points).
Make ellipses using the proper character, not periods and spaces.
Apostrophes point downward.
Make sure foot and inch marks are straight, not curly.