The Essay as a Whole
Strunk and White's first three Principles of Composition apply both to paragraphs and essays:
Choose a suitable design and hold to it.
Make the paragraph the unit of composition.
As a rule, begin each paragraph with a topic sentence; end it in conformity with the beginning.
Most contemporary textbooks make the same points under the headings unity, coherence, and development.
I suggest Seven Rules for writing essays.
Sample outlines for essays (for discussion)
Sample outlines for essays on specific texts (for discussion)
Sample essays (on "Multiculturalism vs. Assimilation")
Evaluating essays: some questions to ask about your own and others' work