Here is how you back up your blogger posts.
1- Click on Settings
2- Click Export blog
3- DO NOT DELETE YOUR BLOG!!!!!!
This will back up all posts you have, not just today's posts. Obviously you'd want to back them up from time to time to make sure you get current posts.
OK...how easy was that? You can also import blog posts (which you might want to do if you start a new blog and want to move posts over.) The Blogger Buzz has a more in depth post about exporting/importing posts. Check it out here.