Always keep a recent backup of your site. Backups are very useful for many reasons in addition to the accidents that cause data lost. Joomla sites can be compromised by malicious attackers. Joomla is designed for a team of editors and any one of them can make an error to harm the site. There are many reasons website administrators need to revert the site back to a previous working state.
It helps to practice both backup and restore. Nobody should wait for an emergency to test their skills and tools for dealing with an emergency. Very often people create a backup of something that is empty or corrupted, and discover that none of their critical data was really backed up in the first place. Nobody likes a ship that sinks with empty lifeboats, and it's also wise to practice getting people out of the lifeboats and into a new working system. Backup and restore also help web managers create a staging website where new risky changes can be practiced without risking the production website. Staging sites can be on a local machine acting as a test server, or any other folder or web hosting account supporting the SQL and PHP versions used by the site you've backed up.
There are two parts to a complete back up of your Joomla site. a) The database informormation, most often found in mysql b) The files and folders on your website, most often
One of the first steps to backup your Joomla site is to close the site to public for a few seconds or minutes, backup the files, and then re-open the site. The steps from phpmyadmin.net omit this much needed operation. Go to the backend of your Joomla control panel, under global configuration, under the site tab, set "site offline" = yes. This will then change the look of your configuration.php file in the root of your Joomla website.
Most managers of a Joomla website also have access to their SQL data using the GUI interface called PhpMyAdmin. The link below shows the basic steps to backup a database using PhpMyAdmin (common on most web hosts): http://www.phpmyadmin.net/documentation/#faq6_4
Alternatively, there are several backup extensions for Joomla 1.5 you could look into, Akeeba Backup (formerly JoomlaPack) for a full backup of your site or SQL Dumper that can be used for both creating and restoring Joomla databases, Although if for some reason Joomla becomes impossible to log into, the backup will need to be restored through PhpMyAdmin or another method. both these extensions and more are available at http://extensions.joomla.org/
It is recommended to back up the database at least twice per week or even everyday (and more) if you have an active site.
Besides the DB, you should also back up your site files. The interval for this depends on how often you or your users make changes to the file system. If users can add photos and media or you make changes to the site other than simple text edits, it should be done often. This is best done with an FTP program and is no different than backing up a standard website. Download all the files and folders that exist in the main directory, and save it to a folder on your local computer. Be sure that the file and directory structure remains the same as it is in the live site. You could then burn a CD or save to an external hard drive if you desire.
TIP. If you've never used an FTP client, most hosts provide an FTP capability from within their control panel.