If it is connected to a sisco switch, you can configure a SPAN port, to monitor all switch traffic. Otherwise, you would have to place a HUB to intercept all the traffic.
If you have only one router, you would have to connect the whole LAN to the HUB, and then connect HUB to the router.
The only other trick I know of, is to set up an "unknown MAC" on your PC, so all traffic will be forcefully forwarded to your port.