Question on start of ASP pages with XHTML

Okay, well I develop my sites with ASP, and I want to use XHTML as well, and since it has to be very properly done, how should you set up an ASP page? Usually, at the top of an ASP page, your suppose to declare the language your using: i.e. <%@ Language="VBScript" %>. If it's not at the first line, you'll get an error.

If it's just as simple as to put the <!DOCTYPE> line right below that, then I have another question too: What about the fact that the XHTML language in general is updated often? In the current DOCTYPE tags, you use XHTML 1.0 transitional or whatever your using for your site... but let's say your going to change 1.0 to something else, is it necessary to have to keep going through all the pages of your site to update as necessary? Is it okay to keep the <!DOCTYPE> line in an include file and include it on every page with the <!--#include--> tag, so if need be, you can update the one include file and all your other pages update as well? I understand that with each new version, some changes may need to be made, and I'm aware of that, but is it still okay to put the <!DOCTYPE> line in one file and still include it in all other files on a site?

Thanks in advance.