Sign Out link

Hi all, I've created a Sign Out link on my page, just like the one you see on the top right hand corner of this forum.
This is the code i use :
<asp:HyperLink NavigateUrl="~/Login.aspx" ID="hypSignOut" runat="server" Text="Sign Out"></asp:HyperLink>
However, after signing out, I click the back button on my browser, I can still go back into the site. How do I prevent this?
Thanks in advance for all the help.
...