"Paragraphs are clusters of information supporting an essay's main point (or advancing a story's action)."
Aim for paragraphs that are:
Neither too long nor too short for easy reading
A General Rule-of-Thumb: "One Main Idea Per Paragraph."
A.) Focus on a Main Point.
Stating the main point in a topic sentence.
"As readers move into a paragraph, they need to know where they are -- in relation to the whole essay -- and what to expect in the sentences to come. A good topic sentence, a one-sentence summary of the paragraph's main point, acts as a signpost pointing in two directions: (1) backward to ward the thesis of the essay and (2) forward toward the body of the paragraph." (39)
"A topic sentence is more general than the material supporting it."
"Usually the topic sentence comes first."
"Sometimes the topic sentence is introduced by a transitional sentence linking it to earlier material."
"Occasionally the topic sentence may be withheld until the end of the paragraph -- but only if the earlier sentences hang together so well that the reader perceives their direction, if not their exact point."
"Although it is generally wise to use topic sentences, at times they are unnecessary. A topic sentence may not be needed if:
A paragraph continues developing an idea clearly introduced in a previous paragraph
The details of the paragraph unmistakably suggest its main point
The paragraph appears in a narrative of events where generalizations might interrupt the flow of the story
Sticking to the point.
"Sentences that do not support the topic sentence destroy the UNITY of the paragraph. If the paragraph is otherwise well focused, such offending sentences can simply be deleted or perhaps moved elsewhere."
"Sometimes the cure for a dis-unified paragraph is not as simple as deleting or moving material. Writers often wander into uncharted territory because they cannot think of enough evidence to support a topic sentence. Feeling that it is too soon to break into a new paragraph, they move on to new ideas for which they have not prepared the reader. When this happens, the writer is faced with a choice: Either find more evidence to support the topic sentence or adjust the topic sentence to mesh with the evidence that is available."
B.) Develop the Main Point.
"Though an occasional short paragraph is fine, particularly if it functions as a transition or emphasizes a point, a series of brief paragraphs suggests inadequate development. How much development is enough? That varies, depending on the writer's purpose and audience."
Development / Support / Evidence -- includes:
Personal experiences and observations and knowledge
Shared values and beliefs
Examples -- brief and extended / real and invented (hypothetical)
Details -- sensory / other
Quotations from authorities / experts
Documented information / research
EXPLANATIONS of how the "support" or "evidence" backs up the topic sentence and/or the thesis statement -- the "So What?" question
C.) Choose a Suitable Pattern of Organization.
"Although paragraphs (and indeed whole essays) may be patterned in any number of ways, certain patterns of organization occur frequently, either alone or in combination."
"There is nothing particularly magical about these patterns (sometimes called methods of development). They simply reflect some of the ways in which we think."
Examples and Illustrations
Comparison and Contrast
Cause and Effect
Classification and Division
Other patterns include various forms of argument and proposals.
SOURCE: My Outline for Rules for Writers, chapter 4, pages 39-50