Make items at the same level of generality as parallel as possible.
Use sentences unless phrases are clear.
Use the conventional system of numbers and letters for the levels of generality.
Always use at least two subdivisions for a category, since nothing can be divided into fewer than two parts.
Limit the number of major sections in the outline; if the list of roman numerals begins to look like a laundry list, find some way of clustering the items into a few major categories with more subcategories.
Be flexible; in other words, be prepared to change your outline as your drafts evolve.