
BUTT, BUTT, THEN YU W0NT BE ABLE T0 $EE THE CAR$ $NEAK!G UP BEHIND YU AND
P0P A WHHHEEEEELLLLEEEEE T0 D!TCHUM AND THE CHICK$ DIG IT!

Seriously, though, putting a helmet on a mirror is inadvisable because
you are then resting the weight of the helmet on the fragile foam
liner that is expected to cushion your noggin.  And once crushed, that
foam never pops back.

A variation of mirror hanging thing is what I do; with the bike on the
sidestand (hey, the centerstand is for maintenance) I turn the wheel to
the stop (left) and can hang the helmet by the chin-bar on the right
grip.  There is no crushable foam in the chin-bar, and it is pretty
secure on there (as long as the forks are locked).  If I have to leave
the helmet with the parked bike (and the bags aren't on/full), that is
about the most stable place.