Using subsections to auto-generate tables of contents
Medium and large articles should be broken into subsections using MediaWiki's section heading notation. In addition to establishing a consistent convention for organizing pieces of articles, it also allows for automatic generation of tables of contents - see PBS for an example. If you use section heading dividers in an article, a hyperlinked table of contents will show up automatically, which makes navigation much easier for the reader.
Bulleted and numbered lists
We often need to provide step-by-step instructions or item lists in our documentation. Use MediaWiki's bullet (*) and numbered list (#) notations to denote separate items. If ordering of the list is critical (first do A, then do B), use a numbered list; otherwise, use bullets.
Use code blocks when providing command-line examples or text file snippets. Prepend each line you want to appear in a text box with a space (this goes for blank lines too, if you want to embed a blank line within your code block).
General documentation guidelines
Avoid creating articles that are so long that the reader has to scroll through several screens worth of content - break them into smaller articles. Use your best judgment, though, Goldilocks! If an article consumes a small fraction of a screen, consider whether it could be lumped into another related article.
For screenshots, PNG or GIF files are almost always preferable to JPG's. JPG files are are best reserved for photographs with lots of color detail and result in snowy-looking "JPG compression artifacts" surrounding text. Here's an example of an image that should not be a JPG.
Favor short, bullet-point explanations over lengthy paragraphs. Where a complex issue is covered, consider breaking topics out into "Quick Start" and sections for more detailed/advanced topics.
Avoid writing out email addresses in text, as they can be harvested by spammers' web bots.
For consistency, capitalize all major words in article titles but not article headings and subheadings.