You could use svnadmin dump. For example, to create a compressed backup file in Linux run:
svnadmin dump -q /path/to/repo | bzip2 -9 > repo_backup.bz2
To restore a backup use svnadmin load:
svnadmin create /path/to/newrepo
bzip2 -cd repo_backup.bz2 | svnadmin load /path/to/newrepo
See also Repository data migration using svnadmin in the SVN Book.
