The good news is - this is a simple enpugh task. The Template Editor now includes a pair of scripts to save and restore the template, as needed. It's a bit more user friendly than starting Notepad each time.
Start from the dashboard Template wizard. Look at the upper right corner of the wizard, and hit "Backup / Restore". Hit "Download full template", point the wizard into the right folder, select or type a file / file name, and hit Open.
Select Download full template to Save the template. Hit Choose File to select a file to Restore the template from, then Upload to do the restore. What could be easier?
Before you make any changes to your template, back it up. But whenever you do backup, consider what you'll be able to backup - and how you will restore what's backed up.
If you decide that you don't like your changes, instead of spending time fixing each change, just restore from backup. Short term result - simple, on the fly changes. Long term result - more progress in making changes, and a more pleasing blog for your readers.