I have a SIL and a niece who work for airlines. From what I can tell, they monitor the routes they’re interested in, noting day of week, times seats are available and how popular the route is with fellow employees. Once familiar with the patterns, you just plan accordingly and be very flexible. Sometimes they’ll fly to a new destination just because of seat availability to explore the area. Currently, my brother and SIL are looking for a flight to either New Zealand or Thailand, whichever flight they can fly first class.
Oh, and as a side perk, SIL upgraded my kids on their flights. For instance, she put son and friend in a row with more leg room. The other kids on the trip were all jealous on how they "lucked out" on seat assignments.