Or what's even the easiest solution...
response.redirect("~/yourpage.aspx")
If you are simply clearing everything and starting over then the above makes sure everything is reset.
You can also do this in javascript so as not to make a trip back to the server which so far all the examples require.
Assuming it is not nested in another control:
document.getElementById(
'<%=tbTest.ClientID%>').value = ""
Assuming your textbox is in a formview:
document.getElementById(
'<%= formview1.findcontrol("your_textbox").ClientID %>').value = ""