Blank lines between paragraphs.
Punctuation at the ends of sentences.
One blank line
One and one-half blank lines
Two blank lines
Three blank lines
All information is indented one-half inch.
All information is typed flush left.
All information is double-spaced.
Information is either indented or typed flush left, depending on who the letter is will be sent to.
Writing too long a letter
Typing the name of the person sending the letter in all capital letters
Here's an interesting quiz for you.