We fly mostly on Southwest, and because it is a no-seat assignment airline, it takes a bit of strategy to get a good seat. SW stacks passengers before flights in waiting lines marked A, B, C and D. Boarding passes may be activated on line from 24 hours before the flight.
If you can print out your boarding pass within minutes of the starting time, you'll probably get assigned to the first 30 passengers to board, those with A boarding passes. If the SW flight originates in your city, you'll have the choice of any seat you want. If your city is a stop-over and there are other passengers already seated, at least you'll have first choice of the best available seats.
If you wait several hours to get your online boarding pass or wait to get it at a kiosk at the airport just an hour before flight time, your chances are that it will be a B, C or last-to-board D pass holders.
Another way of getting the best seats on SW is if you are considered special boarding. Families with little kids, people in wheelchairs and crutches, and the very elderly are allowed to board before everyone else.
As for getting the best pre-assigned seats on flights, money talks for top choices. Many airlines now have all kinds of pricey seats up front, called first class, business class or some other snooty title. The seats are plush and wide, are usually just two across, very comfy, with lots of leg room and a steak dinner and champagne thrown in.
Back where the crammed-in peasants sit in five-seats-across rows, they may be paying only $200 for their uncomfy flight, while the up-front royalty could be paying $2,000. Of couse, both arrive at the same destination at exactly the same time.
Most travel agencies, both online and traditional ones, will have complete seat charts for you to consider before you make your reservations on seat-assignment airlines. The earlier you make them, the greater choice of seats. Taste in seat selection varies from person to person.
Many prefer window seats that are near the front of the plane. Some believe a seat over a wing has the least motion for those who fear air sickness. I have a gimpy left leg, so I always try for an aisle seat where I can stretch it out after everyone is seated.
Once seated in a reserved seat doesn't necessarily put glue on your pants. If, during the flight, you have bothersome seatmates, and there are available seats elsewhere, you can ask a flight attendant to get you to another seat. If there are many empty seats, there's no reason why you have to suffer in the middle of a row of five across.
When there are three or more empty seats together on a long flight, and you want to stretch out, as soon as the aircraft is in the air, either jump up and claim them, or when necessary, ask a flight attendant to hold some for you. Whenever I fly cross country or overseas, and have paid only for a tourist seat, I've often asked the attendant if I could sack out forward in an unsold comfy upper-class seat. Sometimes it works, sometimes not.
If you're flying with small kids or infants, you may want to select a bulkhead seat, those facing walls between compartments. They usually don't offer much more leg room, but there is a measure of privacy for diaper changes and other childcare chores. In some airlines, there are hooks holding net hammocks on the walls, and they make soothing, swaying beddy-byes for infants. Additionally, your kids won't be as bothersome to other passengers, because they won't have people sitting in front of them.
The best way to get the best choice of seats, when your airline assigns them, is to make your reservations as early as possible. If you'll be flying Southwest or other airline that offers seats first come, first seated, be sure to keep your computer fired up so that on the stroke of 24 hours before flight time, you can print out your A boarding pass.