An empty seat is the best thing you can do to have a better flight. On most airlines it is luck. On Southwest it is strategy.
Southwest doesn’t pre-assign seats, but if you board early and get the one you want and the flight isn’t sold out, you can improve your odds that the seat next to you stays vacant – by making it unappealing for someone to choose to sit down next to you. Always ask if the flight is full when you board!