All major search engines have guidelines to help get the best possible placing for your site:
Google Information for Webmasters -> http://www.google.com/webmasters/guidelines.html
Yahoo! Guidelines on Spam -> http://help.yahoo.com/help/us/ysearch/deletions/deletions-05.html
MSN Guidelines for successful indexing -> http://search.msn.com/docs/siteowner.aspx?t=SEARCH_WEBMASTER_REF_GuidelinesforOptimizingSite.htm
Teoma Tech Information (for Ask) -> http://sp.ask.com/docs/about/aj/teoma.htm
