Dim the interior lights. On a North American Prius, there is a dimmer wheel on the dashboard to the left of the steering wheel. All the way up is full day brightness, all the way down is no interior/dash lights. I usually keep mine set a smidgen above the completely dark setting (so just barely lit), because the glare will give me motion sickness headaches.
However, on the 2004+ Prius, there is a little area above the combination meter where some light will bleed through a seam, and that can be difficult to control. Some black tape or a black fabric placed on top of this seam usually works for those people who are sensitive to this thin strip of light glare.