If the flight is sparse, I head to the back with lots of chances to get at least a middle empty if not all three seats to myself. People sit next to each other in the front no matter how full the plane is. For full flights, I like to get about 8 rows back. That's where the overheads will be full and all the last people on the plane will have to keep heading to the back, so best chance for a middle seat to be empty is in that neighborhood. I've heard it called "The Hidden First Class" of Southwest. Also for short flights I forget which row exactly, but you want to be towards the first group to get drink orders in, but not for the front FA who has some extra duties, get the second section.
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