Use javascript to clear all fields when value in dropdownlist changed.
<script type="text/javascript">
function onValueChange()
{
try {
document.getElementById('<%=TextBox_Name.ClientID%>').value="";
}
catch(e){alert(e.message);}
}
</script>
Call this function on
onchange='onValueChange();'
event of dropdownlist.
Must add
ClientIDMode="Static"
attribute to textbox which you want to make clear.