The problem here is that you must be somewhere along the path, sniffing these packets. I actually do this at my office as part of my normal routine, checking for specially crafted packets that might be unwanted in my network!
In my setup, I have placed a small hub between the firewall and router, so I am able to capture and trace all packets with ease. The nature of hubs, allow you to read all data without any special actions required, where as with switches, it's a different story.
In addition, most serious packet sniffers will allow you to save the data captured so you get the file transferred conveniently placed in your hard drive!
A quick search in Google will surely reveal many sites and programs that contain some good information on how to do this!
This was first published in April 2004