How to Remove Your Google Search History Before Google's New Privacy Policy Takes Effect

https://www.eff.org/deeplinks/2012/02/how-remove-your-google-search-history-googles-new-privacy-policy-takes-effect
It is important to note that disabling Web History in your Google account will not prevent Google from gathering and storing this information and using it for internal purposes.

On March 1st, Google will implement its new, unified privacy policy, which will affect data Google has collected on you prior to March 1st as well as data it collects on you in the future. Until now, your Google Web History (your Google searches and sites visited) was cordoned off from Google's other products. This protection was especially important because search data can reveal particularly sensitive information about you, including facts about your location, interests, age, sexual orientation, religion, health concerns, and more. If you want to keep Google from combining your Web History with the data they have gathered about you in their other products, such as YouTube or Google Plus, you may want to remove all items from your Web History and stop your Web History from being recorded in the future.

More information in the article.