Force users to complete form in certain order

Hi
How can I force users to complete a form in a certain order.
Something like this - so they must complete DDL1 before DDL2 and then DDL3, etc Can this be done with a message to ask them to complete the previous DDL if they try to select from the next. Thanks
J

<form id="form1" runat="server"> <div id="MyDiv1"> <asp:DropDownList runat="server" id="DDL1"> <asp:listitem></asp:listitem> <asp:listitem>ABC</asp:listitem><asp:listitem>XYZ</asp:listitem></asp:DropDownList> </div> <br /> <div id="MyDiv2"> <asp:DropDownList runat="server" id="DDL2"> <asp:listitem></asp:listitem> <asp:listitem>ABC</asp:listitem><asp:listitem>XYZ</asp:listitem> </asp:DropDownList> </div> ...