<asp:ListItem Text="SP" Value= "1" Selected="true" ></asp:ListItem>
Update :
Before leaving the page, you can store your selected value in a Session.
Session["SelectedListItem"] = ListBox1.SelectedValue;
And on your PageLoad, you can check the
Session["SelectedListItem"]
if it has value and set it.
if (Session["SelectedListItem"] != null) {
ListBox1.SelectedValue = (string)Session["SelectedListItem"];
}