As you organize your argument, concentrate on limiting each paragraph to one main idea (which has been introduced in your topic sentence).

Making Paragraph Breaks (Sentence Sense)
Paragraphs (Length Considerations) (Purdue OWL)

The Topic Sentence (Strunk and White)
Paragraph Development and Topic Sentences (Darling et al.)
The Five Paragraph Essay (Darling et al)
Paragraphs and Paragraphing  (Purdue OWL)

Developing an Outline (Purdue OWL)