How to Get a Good Seat on an Airplane

Choose which class you will be traveling in. Try to get an upgrade to first class.
First Class is the most luxurious class. Large seats, plenty of legroom, improved entertainment, and good meals can be expected. First class is also the most expensive, with tickets ranging in the ten-thousands for long or international trips.
Business Class is more or less similar to First Class, though often not quite as spacious.
Economy Class is the least luxurious class and the least expensive. Seats in economy class receive reduced service quality, smaller seats, and average food. Usually, economy class takes up most of the airplane and includes most of the poor seats.
Premium Economy class is available on some flights. Middle seats are usually filled last in this section, possibly giving you some additional room to feel comfortable. Premium economy seats are typically in the economy seating area, but feature a little more legroom, better service, and possibly improved in-flight entertainment. 

Make a list of your preferences, if flying economy class (seats vary in economy class, while all first class typically comes with these features).

Choose which airline you will be flying with, if not already done so. Some are better than others.

Once you find a candidate airline, try to find out which type of aircraft usually serves the route you will be taking. Is it a Boeing 767-400ER? Or is it an Airbus A340? Make sure that you are specific (A 777-200 is different from a 777-300).

Check an online seat guide like SeatGuru,Seat Expert or USATODAY's seat comfort reports  to find out which seats...

Ask for an exit row seat. They usually have additional pitch (which is the distance between the rows, measured from the back of one seat to the back of the seat behind) and legroom, but you have to be capable and willing to help in case of an emergency. Additionally, the seats in front of the exit row don't usually recline, giving you even more room than normal.

Strategize for an empty middle seat. Look for an aisle seat towards the rear of the plane where someone is already sitting in the window seat (or vice versa). The reason you want a row where there's already someone in the window or aisle seat is because if you sit down in a row with two empty seats next to you, there's a good chance that a traveling pair will take them.

Know which planes have the best seat pitch.Sometimes even just an extra inch of pitch can make a big difference in terms of comfort and, say, your ability to work on a laptop.

Book your flight. It's best to book online, directly from the airline's booking site, as you can usually easily pick which seat you want.

On the day of your flight, get to the airport with plenty of time to spare – if you are not checked in on time, the airline may give your seat to another passenger. Plan on arriving at the airport at least two hours before an international flight and a one and one half-hours before a domestic flight to allow time for check-in and security.

Enjoy your flight with your good seat!