pinning R packages to unstable?

Dear Debian R users,

This is more of an apt question rather than R.  I have a testing Debian
system, but would like to use the unstable packages for R, *without* using
the -t flag in apt-get.  I think this is done by using a 'pin' rule in
/etc/apt/apt.conf, but don't understand how this should be done.  Can
somebody provide an example please?


Cheers,

Seb