Here's a useful little trick if you want to try out and debug a new template on your site while your visitors still see the live template.
joomla1.0.x : Simply use this link "index.php?jos_change_template=new_template_name"
joomla1.5.x: Simply use this link "index.php?template=new_template_name"
Please see my examples at demo10x.joomlavm.com and demo15.joomlavm.com