Next time, try this. Whenever you get on board, go directly to whatever seat you want. If someone else is already there, and you want it bad enough, make whatever deal you have to in order to get them to switch. After all, it is "open seating" and you both have the same right to the seat.

Have something prepared in advance to offer in exchange for the switch if it's really that important. Not that important?, then sit somewhere else.