In short, paragraphs should exhibit the same structure as whole sections. A reader should be able to grasp the thrust of your argument by reading only the first sentence of every paragraph. Write short, declarative sentences. Avoid the passive voice. Always write from an outline.
Outlines are major aids to coherence and readability.
For more advice on writing see William Strunk Jr. and E.B. White, The Elements of Style , 3rd. If you are doing a research paper, you might also consult Kate L. Turabian, A Student's Guide to Writing College Papers , 3rd ed. Two heads are better than one, and giving and receiving comments are important skills. Take care to turn in a neat, clean paper. Run your spellchecker.