There is already a resource page on backing up template. This is my version: Backup and edit New Blogger template. 

I will advice you to also backup your Page Elements. This is because whenever you change template, Blogger will likely delete some Page Elements or some of them can become empty, doing nothing. When you backup your Page Elements, after putting in a new template (can be your backup template or a fresh template), you can easily add back those Page Elements that had been deleted or become empty. Not all Page Elements can be backed up. Some like the Profile, Archives, Labels, cannot be backed up. However, this is not a problem as this can be added back again direct from your "Add a Page Element" in the LAYOUT. 

Peter @ Enviroman (Blog*Star)

Blogger Tips and Tricks