Install this on/off valve between your existing showerhead and the showerarm and you'll be able to temporarilly interrupt the flow of water during a shower while you lather. Push the valve one way and the flow will cease. Push it the other and the flow will resume.
In addition, this device has a maximum flow rate of 2.0 gpm, so even if you have a older "water guzzling" showerhead, you'll realize savings even while the water is running. It is made of chrome plated solid brass.