Using the parking brake.
It happened to me once many years ago. After push-back the tug was disconnected without telling me, so, as the apron had a noticable slope, the aircraft started rolling backwards. At this point I already had my hand on the parking brake handle waiting for the signal from the ground to set it.
For some reason I looked out the side window and notice we were moving backwards and, as a pure reflex, pulled the parking brake handle. I can tell you as the nose came up there was a lot of 'Oh crap' being recorded on the CVR.
It seemed to take forever for the nose to drop again. A very uncomfortable feeling I can tell you.