Backing up your blog

To display all your posts in one page write:
http://blogname.blogspot.com/search?max-results=N
(N:number of posts)
Then you can use HTTrack or any similar application.

For an XML feed write:
http://blogname.blogspot.com/feeds/posts/default?max-results=N

Backing up comments:
http://blogname.blogspot.com/feeds/comments/default?max-results=N