<asp:TextBox ID="txtName" ClientIDMode="static" runat="server"></asp.TextBox>
References :
http://weblogs.asp.net/scottgu/cleaner-html-markup-with-asp-net-4-web-forms-client-ids-vs-2010-and-net-4-0-series
http://stackoverflow.com/questions/3304478/how-to-set-specific-id-for-server-controls-in-an-asp-net-web-form-that-is-using