Click on Design in the upper right-hand corner of your blog window.This was something I had been meaning to do for years, but kept postponing. TYWKIWDBI has an archive of over 9,000 posts and 25,000 comments, so I'd hate to have it disappear.
The new window that comes up will have a list on the left. Click on the last one: settings.
Then click on other at the bottom of the list.
Towards the top of the page, you will see Blog Tools; click on Export Blog.
Don't worry, this will not make your blog go away, but it will create a file that includes all of your posts and the comments too!
The backup process took me less than five minutes, and used up only 80 MB on a thumb drive.