Pass label or textbox value using Image Button

 I have a label control and an image button on a page. I want to be able to pass the value of the label control in a querystring when the image button is clicked. I have setup the postbackurl for the image button, but how do I make this dynamic so that it includes the label value?
<asp:Label ID="pubid" runat="server" Visible="False"></asp:Label><br /> <b>Publication Name:</b> <asp:Label ID="pubname" runat="server" Visible="true"></asp:Label> <br /> <asp:ImageButton ID="btn_pub" runat="server" AlternateText="Publication Submit Button" Height="30px" ImageUrl="~/Images/publication_button.jpg" Width="115px" PostBackUrl="~/Publications.aspx"/>
& ...