R – Detect page is loaded into Sharepoint Designer

from: http://office.microsoft.com/en-us/sharepointdesigner/HA101513941033.aspx

Insert a Web Part zone

1. In Office SharePoint Designer 2007, open the page where you want to insert the Web Part zone.
2. If the Web Parts task pane is not already open, open it by clicking Web Parts on the Task Panes menu.
3. In Design view, click the location on the page where you want to insert the Web Part zone.

4. At the bottom of the Web Parts task pane, click New Web Part Zone.

   The new Web Part zone is inserted on the page. ...

I do the same for small changes and it has worked fine for me up to now. That said the only offical way to do it is use Microsoft's Data Protection Manager software which will let you backup/restore individual pages.