Always look both ways on a one-way street. I've seen other drivers drive the wrong way down one-way streets. I've also accidentally done it when the signage was non-existent where I got on the street.
Growing up in NYC you were taught to stop and look both ways no matter what. It's not always a car going the wrong way. It could be a cyclist, skate boarder, roller blader, or some idiot on a moped or other wheeled motorized thing.