There's nothing more devastating in the world of computing than not having a backup of your data when something goes wrong, and the same is true on the Web. Hard drives die on web servers and even the most sophisticated server backup systems are vulnerable to human error or acts of nature.
Your hosting company may offer automated backups, but double-check exactly what that means. If it's just your WordPress files, it's only half a backup. If your database and all necessary WordPress files are getting backed up, be sure that you can get access to those backups and download a copy to your computer for additional safety.
Here's one simple routine for backing up your WordPress site. In the folder on your hard drive where you have anything relating to your site, create a new folder called My Site Backups. Then pick a time each month (more frequently if you're constantly adding new content) when you're going to stop for a moment and back up your site.
Each time you do the backup, create a new folder in My Site Backups using a dating system such as 20110617 (year-month-day). This naming system makes it easy to find, whether you need the latest backup or if you want to restore the site from an earlier time.
Backing up WordPress is a two-step process, so in this 20110617 folder, you're going to have two subfolders: one for the database backup and ...