Add a new ground line from existing ungrounded receptacles, and run it back to the grounding bar at the fuse box. Inside the receptacle, attach the wire to the box using a green screw. If the existing wires are run through metal casing, the casing serves as the ground.
If running wires back to the grounding bar is not possible, a ground fault circuit interrupter, or GFCI, is used. This method does not add a ground but faults in the event of a short circuit. Faulting prevents additional current from flowing, neutralizing the thread of electrical shock. These circuits must be labelled "No Equipment Ground."