Create your site (any level) and once you are happy with , change URL in your browser to: http:/server/sitename/_layouts/savetmpl.aspx
You will now be given an option to save the site as a template.
Don’t forget to choose "Include content" option if you wish to retain content you’ve loaded into the site.
The new template will appear in Site Template gallery and will be available on every level of the portal.
There is a downside. The template appears in the Collaboration tab (when you Create Site). I couldn’t figure out how to move it to another tab. I guess the template is loaded into database and the system gets information about it from the DB. If you know how to move it let me know.