I commute from a city that is sometimes "full" in the back with jumpseaters. I just email the Capt. and ask for his approval to ride "up front." I've never had anybody say no and I've only heard of a few stories where capt's have a problem with it.

Emailing the Capt. will also get you head of the line cockpit privileges. I don't believe in the FedEx tradition of bringing cookies but I usually will if I'm seated in the cockpit.