There's probably a quicker and easier way, but if you type in each letter/numeral (and presumably symbol, if you have searched on them), a drop down list appears. Go down and press delete to remove each item individually. If you use the Google Toolbar (though there MAY be privacy issues with this - see an earlier question) if you press the "down" arrow to the left of the search box, you can select "clear search history" from the options. This does not remove queries typed directly into Google though - only those typed into the Toolbar search box.