Steven,

If it is just a rectangular board that they are going to shear off you
could put dummy traces in the corners or around the outline and tell the
board house to shear along them.

A better way, and the only way if the board is going to be routed
instead of just sheared off would be to make a seperate board outline or
mechanical layer that shows the outline.  Send it to them as a seperate
gerber file.

When you send your gerbers you will always want to include a readme file
with names of the file for each layer specified.  In this file you can
specify which gerber is your board outline and give any other special
instructions.

John Ellenz