The Stanford guidelines for web credibility:

• Make it easy to verify the accuracy of the information on your site.
• Show that there's a real organization behind your site.
• Highlight the expertise in your organization and in the content and services you provide.
• Show that honest and trustworthy people stand behind your site.
• Make it easy to contact you.
• Design your site so it looks professional (or is appropriate for your purpose).
• Make your site easy to use -- and useful.
• Update your site's content often (at least show it's been reviewed recently).
• Use restraint with any promotional content (e.g., ads, offers).
• Avoid errors of all types, no matter how small they seem.