Just to add my two cents worth:

You can get several strengths of signal out of your traditional Progressive Electronics typr of toner.

Weakest: Hook up the red and black leads as you are used to doing. One note: you won't get much of a signal if the pair(s) of wires you are connected to are shorted together.

Strong: Hook up the red lead to your wire(s) and leave the black unhooked. Note the tips of others in this thread. (Using the shield on shielded wire, different pairs of cat5, etc.)

Stronger: Hook up the red lead to your wire(s) and ground the black lead. (Your assistant's body is an excellent ground. I usually will have someone keep a thumb on the black lead if I need to do this.)

Keep in mind that when using the strong or stronger, the toner's signal can bleed into other wires near it.

As RTI indicated, try to get rid of as much AC noise as possible while tracing your wires.

Progressive also makes a variable toner that I've had very good luck with. Model 508s [Link: 198.247.193.8]

Happy toning.