Being a long time Wordpress user for my blog (http://www.eclecticguy.com) I use an automatic database backup plugin that emails the backup to me once a week. With my store, it is even more critical to have frequent backups. 

Here's the plugin that I use site: http://ilfilosofo.com/blog/wp-db-backup/

You can do an automatic plugin install in WP by going to Plugins->Add New  Search for "WP-DB-Backup" and this one should appear first in the list. Click the install link and that's it. You do need to configure the backups – listed under Tools.

cheers,

Michael

I use this plug-in and have a Gmail account setup to receive the backups which are done twice each day for my other blog.  I think it's better to be somewhat paranoid and have more information than you need.  By using the Gmail account, I'm not clogging up my inbox with extra messages.  I usually log into the Gmail account and delte all the backups except for the latest backups for the past three or four days.

Also remember – the backup of the database fi;es is only one part of the process.  You also need to backup all the files for your store.  The best bet is to use your FTP program (FiuleZilla is a free version) to copy all the files to your hard drive from your site, zip them up (Trial version of WinZip is available) and E-mail them to your Gmail account.  This way if something does happen to your store all your files as well as your database backup are available offsite for you to retrive and reload.

NOTE:  Be sure to select all the database files within the plugin settings.