I think Flygirl means at LAX don't even try to get it for takeoff, but after the flight is in progress you can ask (doesn't hurt to ask, I've been up front before on US-Canada flights, but policies may have changed since that time). Also, if you're really nice to the F/A's, and you wait towards the end of the flight (wait a little bit after the meal is served) it improves your chances at getting the jumpseat for landing. If you do get it for landing, be sure to respect the pilots by not interrupting, asking questions, or distracting them when they're busy. They're usually very happy to talk when they can, but they have a job to do.
BTW, when you're in Calgary, make sure you get a milkshake at Peter's Drive-In on 16th Ave NW