Get the body text right before you worry about the rest of the document.
Point size should be 10–12 points in printed documents.
Line spacing should be 120–145% of the point size.
The average line length should be 45–90 characters.
Page margins should be larger than one inch on 8.5″ × 11″ paper.
Avoid system fonts if you can.
Use curly quotation marks, not straight ones (see straight and curly quotes).
Use bold or italic as little as possible, and not together.
All caps are fine for less than one line of text.
Use centered text sparingly.
Put only one space between sentences.
Kerning should always be on.
hyphens and dashes are different characters.
Use proper trademark and copyright symbols—not alphabetic approximations.
Use a sequence of underscores to make a signature line.
Apostrophes point downward.