document.getElementById("id_of_selectbox").selectedIndex = -1;
Another and better option is to create a simple function say resetDropbox and call it where-ever you want in your page.
document.getElementById(id_of_dropbox).selectedIndex = -1;
Sometime we face problems, when press back button in browser and we get some value pre-selected in dropbox.
We can use binding reset scripts on jQuery’s Dom Ready, that will put dropbox of the form in its normal(unselected state).