DropDownList Not keeping last value on PostBack (not bound from DB but static value DDL)

This would be a simple one for me if I was binding this dynamically.  I don't think it's necessary to have a code behind for only 4 or so values.  Anyways here is my ddl.
<asp:DropDownList ID="page_size" runat="server" AutoPostBack="true" OnSelectedIndexChanged="page_size_SelectedIndexChanged"> <asp:ListItem Value="10">10</asp:ListItem> <asp:ListItem Value="15">15</asp:ListItem> <asp:ListItem Value="20">20</asp:ListItem> </asp:DropDownList>
 
The problem is that when a user selects a value and it postbacks my ddl resets to the first value.  How do I prevent this?????
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