Here are some guidelines for design of individual Web pages (which you should ignore when appropriate):

General:

• Keep it short and simple
  
• Minimize file size per page to maximize download speed

Page Size:

• Avoid pages and graphics more than 600 pixels wide (which are hard to view on laptops and smaller monitors without scrolling right and left)
• Avoid pages and graphics that are more than a few screens high unless it's important to keep information together (People tend to lose interest the more they scroll down. Also, if they want to print, shorter pages allow them to be more selective about what they print.)

Text:

• Miminize text and eliminate whatever's not critical
• Keep text in columns no more than about 10 words wide, like this one (wider columns are much harder to read)
• Divide text up so that pages don't look dense
• Avoid more than about 50 words or two three lines in a paragraph. (Check how the longer paragraphs look on this page.)

Text Styles:

• Avoid italic because it's often hard to read and doesn't stand out
• Use boldface sparingly, or it loses its impact
• Use physical styles (plain, bold, italic, teletype) rather than logical styles (strong, emphasis, citation, sample, keyboard, code,variable) for greater control

Formats:

• Avoid the preformatted format unless it's really critical or needed for emphasis
• Don't use the Address format because it is often hard to read or too small
• Neither the preformatted nor address format can be used with HTML bullets

Headings:

• Use a larger heading style for the main heading on a page and smaller styles for subheadings
• Don't use headings for emphasis for text elements that are not headings

Links:

• Don't use too many links: only those that really enhance the page and provide value to the visitor
• Integrate links into the text, and avoid "Click here"

Graphics:

• Keep graphics to a minimum file size to reduce downloading time

Backgrounds:

• Make sure there's contrast between the background and text so that text is legible
• Avoid backgrounds that are bright or busy or detract from your content (unless you want that effect)

Also see WebResults' suggested principles for designing Web sites and Web site design process.