Identify the type of aircraft being used on the flight. Look over the seating chart to determine your preference. Some prefer aisle, some window, but you must. decide which is more comfortable and convenient for you. 

Make your requests early, at the time of booking.

Consider joining the airline’s frequent flier mileage program. Often better seats are held out for the airline’s VIP or mileage club members.

Usually, seats on or near the wings will provide a smoother ride.

Remember, bulkhead seats provide more legroom, but often lack overhead compartment space and usually have no under seat storage in front of them.

Often airlines put parents with small children and babies in the bulkhead seats to provide extra room. If you're travelling with have kids, this may be a desirable place for you. Check in advance to determine whether baby seats, bassinets, diaper bags etc. count against your allowable carry-on bags. Keep in mind that small children, will not be seated in an emergency exit row.

To avoid being seated next to babies or small children, ask for the emergency exit row. These rows have more legroom and will have an overhead compartment. In some cases, the seats may not recline. Remember, the airline will ask for and expect your help in opening the emergency exits if an accident or emergency occurs.  Many airlines will not pre-assign the exit seats. You can wait until you reach the check-in counter and make your request then.

Be aware that the seats immediately in front of the exit row will not recline. They have standard legroom, but are better left for someone else who doesn't mind sitting upright through the entire flight !

If it is important for you to deplane as quickly as possible on arrival, request a seat in the first 6 or 8 rows. Those will be the first off the plane.