Whichever type of repository you use, it is vitally important that you maintain regular backups, and that you verify the backup. If the server fails, you may be able to access a recent version of your files, but without the repository all your history is lost forever.

The simplest (but not recommended) way is just to copy the repository folder onto the backup medium. However, you have to be absolutely sure that no process is accessing the data. In this context, access means any access at all. If your repository is accessed at all during the copy, (web browser left open, WebSVN, etc.) the backup will be worthless.

The recommended method is to run

svnadmin hotcopy path/to/repository path/to/backup
    

to create a copy of your repository in a safe manner. Then backup the copy.

The svnadmin tool is installed automatically when you install the Subversion command line client. The easiest way to get this is to check the option to include the command line tools when installing TortoiseSVN, but if you prefer you can download the latest version of command line tools directly from the Subversion website.