Despite it’s lack of frills Southwest is one of the better domestic carriers. The only annoying thing is the stupid cattle-call boarding process getting the general public to behave like the sheep that they are.
Getting to the point; my advice when flying Southwest is to set a calendar reminder in your phone when you book your flight to remind you to check-in online exactly 24 hours before your flight departure. Southwest does not automatically send out an email reminder like other airlines but if you’re smart and check in right at the maximum allowable time (24 hours) you’ll get an ‘A’ boarding zone essentially giving you free rein on where you’d like to sit.
An addendum to this is if you’re in no rush to exit the plane on your arrival, and you miss getting an exit row seat, choose a seat near the rear of the aircraft. These are the last to fill up and you’ll have more of a chance of having the middle seat empty.