Don't use all your fonts in one document.
While it's tempting to use all those pretty fonts, it can be distracting to the reader and you probably won't get your message across. Try sticking to different fonts within one typeface, or one font for text and one for headings.
Don't put in a paragraph space by pressing enter twice.
Almost all word processing programs ? and definitely all page layout programs ? allow you to choose how much space you want after a paragraph. Use this instead. You're more likely to get a consistent look (not to mention even paragraph spacing) using this method.
Don't use two spaces after a period.
Two spaces after a period is a holdover from the days of monospaced fonts, like Courier and typewriters. They helped signal a pause. With proportional fonts, it's unnecessary and can make text downright hard to read.
Don't use straight quote marks.
Page layout programs, and most word processing programs, give you the option to use "curly" quotes. These may also be called smart quotes or typographer's quotes.
Don't leave large text unkerned.
Page layout programs allow you to manually kern your text (increase or decrease space between specific characters). This is a good idea, especially as you use larger fonts (around 24 points and above), since the spaces between characters tend to be more pronounced the larger the font size. In word processing software, you normally can't manually kern text, but you can turn kerning on over a certain point size. Kerning in word processing programs isn't normally on by default; I suggest you find out how to turn and on ? there's no reason not to let the software kern what it can.
Don't center large amounts of text.
Centering is fine for a few words or a headline. But it makes paragraph after paragraph of text very difficult to read. It's hard for your eye to find where the next line begins.
Don't use all capitals – especially if you're using a calligraphic font.
Nothing screams amateur like a headline set in all caps in a calligraphic font. Upper and lowercase letters help people read faster. All capital letters should be used sparingly.
Don't leave widows and orphans.
A widow is a paragraph whose last line is at the beginning of a column of text. An orphan is a paragraph whose first line starts at the bottom of a column of text. You can usually set how many lines must be in the beginning of a paragraph of text, or how many lines must be at the top of a column of text.
Don't use every box and line available in your software.
You look at your document. Looks a little boring. A light bulb goes off! "I'll just throw in some rules and boxes; that'll liven things up". Overuse of rules and boxes only confuse the reader; they don't know which information is the most important, and it makes it hard for readers to skim for the information they're interested in. Use boxes and rules sparingly.
Don't rely on your software's automatic leading feature.
Leading is the space between lines of text. Many programs default to a straight percentage for leading, often 20%. So 10 point text would always be set with 12 point leading, 12 point text with 14 point leading, and so on. But all fonts are not created equal: longer lines of text need more leading; sans serif text usually needs more leading; fonts with a large x-height may need more leading; and headlines typically need less.