How to pass a value from one website to another website

I am using Visual Studio 2010. An external website needs to pass a value to our website (using ASP.NET). What is the best way to do that ? I prefer not to use Query String to pass the values in between the 2 websites, because that will show on the URL address. I read that I can pass values to another site using postback url property of a button. How can I do that ? 1. Does the external website need to create a button like below ?  <asp:button id="Button2" text="Post value to another page" postbackurl="http://myWebSite/myPage.aspx?myValue=123" runat="Server" onclick="Button2_Click"></asp:button> Then, how can I retrieve that value in my website ? 2. Can I use PreviousPage property (but the previous page is the external website), and get the value of a control in the previous page from ...