For years I swore I'd never fly Southwest. I referred to their boarding as cattle call and hated the fact that I couldn't select my own seats. Funny how the changes at Frontier, my favorite airline for almost 15 years, made me a Southwest fan.
Southwest offers a variety of fare options including their "Wanna Get Away" fare. Like all specials, there are a limited number of seats available and once gone, full fares are your only options. If faced with that, check out the business fare. Sometimes it is only $20-$30 more and you are guaranteed a spot between A1 and A15.
If you purchase an inexpensive fare, but don't want to chance being in group B or C, check out Early Bird check in. For $12.50 per segment the computer will check you in 36 hours before the flight - 12 hours before the rest of the world - and usually it will land you in group A. For those of us who didn't want to give up selecting our seat (aisle for me), this option generally gets you in Group A and on the plane early enough to get your choice of an aisle or window seat.
Like all airlines these days, book as far ahead as you can to get the best prices. I sign up for email notices for all the airlines on which I might fly. But once you get the email, don't delay. Specials advertised on those emails disappear fast.
Finally, changes on Southwest. They advertise no fee for changes, which is accurate, but as with all airlines, you have to pay the fare difference. If the change is close to the date of your flight, there probably won't be any "Wanna Get Away" fares available so you leap to full fare, which can be $200-$300 more per seat than you paid originally. Southwest also offers upgrades at the airport. All an upgrade at Southwest means is moving from whatever group you are in to Group A. The problem with their upgrade is it is on top of a "full fare". If you bought a "Wanna Get Away" seat, to upgrade you have to pay the difference between your original fare and the full fare THEN pay the upgrade fee. I don't know about you, but moving to Group A is not worth another $100-$300 per seat.
The last "gotcha" to be aware of is if you notice something missing on your boarding pass. An example of this is a friend who paid for TSA Pre ($85/year) and enters his number when buying a ticket on Southwest. He entered his number at purchase but when he got his boarding pass in Group A, he noticed there was no TSA Pre listed on it. He called Southwest to have it added, which they did, but when his boarding pass reprinted, he was suddenly in Group B - like B42. What Southwest told him was any change made after he checked in basically re-checked him in and put him in the queue with everyone else. So consider this if you make any last many changes/additions.
Overall Southwest is a great airline to fly. Customer service staff and cabin crews are great. Service in flight is good. Southwest is one of the few "economy" airlines that still gives you a drink (more than just water) and a snack during the flight for free. And so far they have not added more rows to their planes to pack in more passengers - you still have some leg space and a real seat try to put things on.