The best solution for packet sniffing (though it only works for actual iOS devices, not the simulator) I've found is to use rvictl. You might have to set the proxy on your local Mac to your instance of a proxy server in order for it to intercept, since the simulator will make use of your local Mac's environment. I'm not sure how you would convince the simulator to talk to it. A man-in-the-middle proxy, like suggested by other answers, is a good solution if you only want to see HTTP/HTTPS traffic.
