Trying to partition a Boot Camp partitioned drive

I am trying to create a new mac partition on a hard drive I have already partitioned using Boot Camp. When I try to do that I get this error message:
The disk appears to be partitioned for Boot Camp. The proposed changes may make this disk using Windows unbootable."

How can I partition it so I wont do anything harmful to the Windows partition?

Thanks
Joel Simkhai

Here's one way:

http://www.coriolis-systems.com/iPartition.php

Regards.