How to add jQuery reference inside a skin file

Of course, you can just include a javascript tag directly in the skin file. - Problem, though - if you use some skin object that requires jQuery (for example, Data Springs DropDown login), it will try to utilize it before the library will land on the page.

Another way to do it, is to put it into Page Header under Page Settings. But that's plain dirty. Hard to maintain, hard to remember, hard all sorts of things. 

Regardless, really, the only clean (read, complaint) way to include javascript on the page is to put it into the head section of the page. in DNN 5+ there is a programmatic way to include this library. It will also make sure you are not going to end up with multiple references to the library, in the event that some module requests it too. So. 

This is how you do it - include the following code in your skin's ascx file:

  <script runat="server">

     Private Sub Page_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load

          DotNetNuke.Framework.jQuery.RequestRegistration()

     End Sub

  </script>