When we go over in class what to do for an assignment, and then people make mistakes that we have covered it disappointing.
Here are frequent mistakes that lost points on the letters:
Not full left-block
Inside address mistakes (wrong information, wrong order, too much information)
Abbreviations 1/17/2009 is an abbreviation for January 17, 2009.
Comma rather than a colon after the salutation.
Too informal "Dear George:" rather than "Dear President Bush:"
Let me stress how important it is to follow formats when they are given to you. Read the rubric for what you will be graded on.