XML site maps for Yahoo
 

I was designing a product website for my own business when a web expert friend of mine suggested me that I should include a XML site map to my website. She said that its one of the SEO techniques and that can help my website to get a high PR in search engine result pages. I did that and it worked for me in Google search engine. I reached number 3 position in a week. However, the same techniques are not working in Yahoo search engine where my rank is 9th. I have no clue why is this happening? Should I bring some changes to the XML site map?