The Basics of Non-Profit SEO, Part 8: If you want to know what the search engines want, ask them

Following these guidelines will help Google find, index, and rank your site. Even if you choose not to implement any of these suggestions, we strongly encourage you to pay very close attention to the "Quality Guidelines," which outline some of the illicit practices that may lead to a site being removed entirely from the Google index or otherwise penalized. If a site has been penalized, it may no longer show up in results on Google.com or on any of Google's partner sites...

• Make a site with a clear hierarchy and text links. Every page should be reachable from at least one static text link.
• Offer a site map to your users with links that point to the important parts of your site. If the site map is larger than 100 or so links, you may want to break the site map into separate pages.
• Create a useful, information-rich site, and write pages that clearly and accurately describe your content.
• Think about the words users would type to find your pages, and make sure that your site actually includes those words within it.
• Try to use text instead of images to display important names, content, or links. The Google crawler doesn't recognize text contained in images. If you must use images for textual content, consider using the "ALT" attribute to include a few words of descriptive text...
• More guidelines from Google

Yahoo! strives to provide the best search experience on the Web by directing searchers to high-quality and relevant web content in response to a search query.

Pages Yahoo! Wants Included in its Index:

• Original and unique content of genuine value
• Pages designed primarily for humans, with search engine considerations secondary
• Links intended to help people find interesting, related content, when applicable
• Metadata (including title and description) that accurately describes the contents of a web page
• Good web design in general

Unfortunately, not all web pages contain information that is valuable to a user. Some pages are created deliberately to trick the search engine into offering inappropriate, redundant or poor-quality search results; this is often called "spam." Yahoo! does not want these pages in the index...More guidelines from Yahoo.

The following are recommendations that might help Bing's MSNBot (The Bing web crawler, a program that scans websites and indexes their content, such as text, documents, images, and links, for searching.) and other web crawlers (A program that scans websites and indexes their content, such as text, documents, images, and links, for searching. The web crawler used by Bing is also known as MSNBot.) effectively index and rank your website. Bing has also provided a list of techniques to avoid if you want to make sure your website is indexed...

Technical recommendations for your website: Use the following techniques to ensure your website is technically optimized for MSNBot and other web crawlers:

• Use only well-formed, HTML code in your webpages. Make sure that all paired tags are closed, and that all links open the correct webpage. For information on validating your HTML code, see either HTTP Compression and HTTP Conditional Get test tool or W3C Markup Validation Service or use a comparable tool.
• If your website contains broken links, MSNBot might not be able to index your website effectively, thus preventing people from reaching all of your webpages. For information on finding broken links on your website, see the Help topic for the Webmaster Center's Crawl Issues tool.
• If you move a webpage, set up the webpage's original URL to redirect people to the new webpage. Indicate whether the move is permanent or temporary. For more information, see What to do when your website is relocated.
• Make sure MSNBot is allowed to crawl your website and isn't on your list of web crawlers that are prohibited from indexing your website. For more information, see Control which webpages on your website are indexed.
• Use a Robots.txt file or tags to control how MSNBot and other web crawlers index your website. You can use the Robots.txt file to prevent web crawlers from crawling specific files and folders. For more information about the Robots.txt file and the Robots Exclusion standard, see A Standard for Robot Exclusion. This site might be available in English only.
• Keep your URLs simple and static. URLs that are complicated or that change frequently are difficult to index as link destinations. For example, the URL www.example.com/mywebpage is easier for MSNBot to crawl and for people to type than a long URL with multiple extensions. Also, a URL that doesn't change is easy for people to remember and bookmark. That makes your webpage a more likely link destination from other websites.
• Watch for malicious software (malware). Links to webpages on your website that lead to malware on third-party websites or contain malicious content, such as a maliciously corrupted image or document file, or a harmful ActiveX control or JavaScript, will be disabled and highlighted as Malware in Bing results webpages. See the Help topics for the Webmaster Center's Crawl Issues tool and Outbound Links tool to learn how to find these detected malware issues on your website. See Remediate detected malware to help rid your website of all malware.

More recommendations from Bing.