I have played this game often and thinking I was in good shape the day before the flight.

Then somehow, a half full plane becomes full.

If flying solo, book an isle close to an empty row, or at least row where you think the seat next to isle in the middle could be empty. Then sit there until somebody comes and claims it. If they do, then go to your original seat. You would of course watch the seat map on your phone - if it is current and make adjustments.