Backing up and restoring a DotNetNuke site is actually pretty easy, so there should be no excuse for not doing it more often.
There are two parts of your site: the website files and the website database. Both parts must be included when backing up and restoring.
1. Backup
Files: Simply make a zip file of your /website directory. Download it using FTP if you are on a hosted server.
Database: Use either your hosting control panel or SQL management studio to backup your database. Select the "overwrite" option, so that you only have one database backup per file. Zip the backup and download it.
Save these two zip files to a secure and seperate location - not on the same computer as the website. A rewriteable CD/DVD is a good option.
2. Restore
Files: Unzip the files backup and copy or FTP them to your /website directory. Overwrite existing files.
Database: Use either your hosting control panel or SQL management studio to backup your database. Select the "overwrite" option.
See its not too hard, so make sure to backup often to avoid the risk of losing vital data. Many hosting companies have automatic backups, but charge you a service fee if you need to have a backup restored. By doing your own backups you can avoid this.