SQL And Proper Concatenation Within VB

Hi all - I have posted inquiries on this rather vexing issue before, so I apologize in advance for revisting this. I am trying to create the code to add the parameters for two CheckBoxLists together. One CheckBoxList allows users to choose a group of Customers by Area Code, the other "CBL" allows users to select Customers by a type of Category that these Customers are grouped into. When a user selects Customers via one or the other CBL, I have no problems. If, however, the user wants to get all the Customers from one or more Area Codes who ALSO may or may not be members of one or more Categories; I have had trouble trying to create the proper SQL. What I have so far:

Protected Sub btn_CustomerSearchCombined_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles btn_CustomerSearchCombined.Click
        Dim CSC_SqlString As String = "SELECT Customers.CustomerID, Customers.CustomerName, Customers.CategoryID, Customers.EstHours, Customers.Locality, Category.Category FROM Customers INNER JOIN Category ON Customers.CategoryID = Category.CategoryID WHERE "
        Dim ACItem As ListItem
        Dim CATItem As ListItem

        For Each ACItem In cbl_CustomersearchAREA.Items
            If ACItem.Selected Then
                CSC_SqlString &= "Customers.AreaCodeID = '" & ACItem.Value & "' OR "
            End If
        Next
        CSC_SqlString &= "' AND " <-- this is the heart of my problem, I believe
        For Each CATItem In cbl_CustomersearchCAT.Items
            If CATItem.Selected Then
                CSC_SqlString &= "Customers.CategoryID = '" & CATItem.Value & "' OR "
            End If
        Next

        CSC_SqlString = Left(CSC_SqlString, Len(CSC_SqlString) - 4)
        CSC_SqlString &= "ORDER By Categories.Category"
        sql_CustomersearchGrid.SelectCommand = CSC_SqlString
    End Sub
Any help on this is much appreciated, many thanks --