Here is a plug-in for Firefox browser designed to prevent Google from tracking your searches.

The plug-in called Google Sharing is an anonymizing proxy service, designed for a very particular need.

It will prevent Google from tracing your searches, what websites you visit and what search results you clicked, etc. This is possible only when you are not logged in a Google service like Gmail, Gdocs, etc.

Remember that Google Sharing is not a complete anonymous proxy service. If you browse a non-Google website traffic will be sent to the internet normally.

You can install this add-on from Mozilla add-on site