1. Check in really early or really last minute
It’s risky. Check in early and don’t get offered it and you have to kill time in the airport; check in last minute and miss the flight and you’ll be kicking yourself for being greedy. However; if you check in first and there are upgrades available on the flight they may offer you the upgrades as you’re the first to arrive. If the flight is overbooked and you’re late but there’s room in business class or above then they might let you on there for the same price. However this only works if you haven’t reserved your seat in economy class on the flight already, and the check in desk are feeling generous as they could also just point blank refuse you entry to the plane.