Ordinarily the body contains more than one paragraph. Avoid both
extremely short and very long paragraphs.
You can use either indented paragraphs (in which the first line is indented
more than the rest) or block paragraphs (in which all lines begin at the left
margin). With block paragraphs you must leave extra white space between
paragraphs -- one blank line or one "empty paragraph" is often used, but
you can also use Word's extra space before or after paragraph option
(Format --> Paragraph...); the extra space should probably not exceed the
size of an empty paragraph. With indented paragraphs, extra space between
paragraphs is common, but optional. Indented paragraphs should be
avoided if the return address was aligned at the left margin.
Special effects like bulleted lists and paragraphs whose left and right edges
are both indented should be used very sparingly -- avoid them as much as
you can. Likewise, consider whether having your paragraphs fully justified
(both left and right edges squared off) will make the letter look too much
like a form letter or piece of junk mail.