History in Search engine

Hi,

How do someone go about clearing the history logs of all the search that he has made on the search engine?

For example, when I go to www.yahoo.com and do a search on dog then the next time when I want to do the search on dog again I just need to type the letter d and it gives me the word dog. How do I clear it so that next time the drop down option doesn't come up ? I tried to delete all the cookies, offline files and clear the history but it still doesn't go away.

I hope I have made myself clear.

Edmond