Every backup plan will vary, but here are my tips on creating a basic backup plan for your blog. Naturally the best option is to create a backup each time you create a new post. For a typical blog posting a few times a week, even a weekly backup will suffice. Another important point to consider is where you store your backups. On most of my blogs, I store a local backup on the server where the blog resides, a backup on my local computer and a remote backup. For the remote backup I use Amazon S3, which is similar to an FTP site, but you can store your backup anywhere that isn't on the same server as your blog.
Here are some references for a few of the most popular blog and CMS platforms:
WordPress If you are comfortable with databases, you will want to backup the database and then backup the blog files. There also is a popular plugin, WP-DB-Backup, which can easily backup the database on a schedule you setup.
Drupal The Drupal community created a module which packages your database and Drupal files together into a downloadable file. On my smaller sites the module works great. On my larger sites, I can't get the module to work. If you try the module and it doesn't complete the backup, check out the Drupal backup guide.
Movable Type Movable Type offers a guide for database backup.
Blogger Alex Chitu put together a simple guide on how to backup a Blogger blog. From his guide I would use the XML options, not the installed software option. The XML option will give you two files: one with your posts and one with your reader's comments.
And most important, always remember to backup your blog files and database before ever attempting an upgrade!