loop through dropdown and set item to be selected

I am filling a dropdownlist depending on some conditions. In the section where make, model, etc are being set, I want to the item selected to be prevfindby which is set at the top of the sub. How is this done?
    Sub a(ByVal sender As System.Object, ByVal e As System.EventArgs)
        Dim prevfindby As String = ddlfindby.SelectedValue
        Response.Write("prevfindby-" + prevfindby)
        ddlfindby.Items.Clear()
        If Not Page.IsPostBack Then
            ddlfindby.Items.Add(New ListItem("Pick a lookup", ""))
        Else
          &n ...