How do I tar a directory without retaining the directory structure?

I'm working on a backup script and want to tar up a file directory:

tar czf ~/backup.tgz /home/username/drupal/sites/default/files

This tars it up, but when I untar the resulting file, it includes the full file structure: the files are in home/username/drupal/sites/default/files.

Is there a way to exclude the parent directories, so that the resulting tar just knows about the last directory (files)?

Use the --directory option:

 tar czf ~/backup.tgz --directory=/home/username/drupal/sites/default files 

Hi I've a better solution when enter in the specified directory it's impossible (Makefiles,etc)

tar -cjvf files.tar.bz2 -C directory/contents/to/be/compressed .

Do not forget the dot (.) at the end !!