The good news is - Blogger now recognises that need. 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. If your blog uses a Layouts template, start from the dashboard, click on "Design" then "Edit HTML". Point the template save / restore wizard into the right folder, select a file / file name, and hit Open.
Select Download Full Template to Save the template. Hit Browse 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. 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.