I have a Debian guest installed on a Windows 7 host using Virtualbox. The installation included virtualbox-ose-guest-utils by default, so I should have everything I need. How can I turn on the shared clipboard?
I'm afraid that you cannot have a shared clipboard on a command-line only guest.
This is because the TTY doesn't really create a proper clipboard that the guest additions can use, you need a full GUI for that.
You will be better off using SSH to log in to the guest from the host. That way you can use the features of your SSH client e.g. Putty to share the clipboard.