Having just read ANOTHER post expounding "insider knowledge" on how to get the best airline seats that did not teach me anything I have decided to share my own actual traveler experience on how to increase your odds to get an optimal seat. No, it won't guarantee a good seat and will not help you if the airplane is packed to the gills, but with a little bit of planning it may work. It can be summarised into two main points: 1) Understand your airlines seating model; and 2) Play the seating game once online check-in opens. As this is from my own experience and preferences which excludes bulk-head seats I am not going to address them in this post, except that the earlier you book your seat the better.
Understand the Airline's Seating Model
Or, work out how the Airline automatically assigns seats to people who do not request one. All airlines fill up economy from the front of the plane. The difference is whether or not they allocate the middle seats before moving to the next row, but what it means to the traveler is that the odds are that you're going to get better seating options towards the rear of the plane. I benefited from this on a flight from Islamabad to Bangkok (there were others, but I can't recall the specifics.) Economy was in 2 cabins, fore and aft. My friend and I selected seats in the aft cabin where on the red-eye flight we had 4 seats each to stretch out and get a good nap. I later found the other members of my party packed like sardines in a completely full front cabin.
Also be aware of other things when initially choosing your seat. Such as:
a certain airline that has it's AV system boxes under the aisle seats which can significantly reduce legroom;
seats with a solid wall behind them may not recline back fully; and
bulkhead and emergency seats may not have removable arm-rests.
Play the Seating Game
This game requires you to initially book the best seat for you, aisle or window, that is towards the rear of the cabin.
It works on the principles of no-one wants a middle seat; and the airline automated seat allocation program.
If you are traveling with another person, book the aisle AND the window seat - and not the middle one. This will significantly address the odds of the spare seat between you remaining unoccupied. If there is someone sitting in the middle seat and you want to give them the window seat to sit next to your partner... I cannot see them objecting to it!
If you are traveling solo, start by booking a seat in an all clear bank of seats - either 3 or 4 seats. Else look for a 3 seat configuration where the aisle/window seat has been reserved and then book the window/aisle seat respectively depending on your preference. In a 4-seat configuration look for places where the aisle seat on the other aisle has been booked. Don't stress too much if you can't find a seat at booking. The pattern will emerge...
Start checking if your seat is still optimal or should you change?
About 12-24 hours after online check-in opens go to online check-in and check how the plane is filling up ... Decide whether or not your seat is still optimal, which for me is a window seat with at least one seat next to me open. Now, here's the key: DON'T COMPLETE CHECK IN OR PRINT YOUR BOARDING PASS. By doing so you "lock in" your seat and it cannot be changed.
Repeat the above step about 12 hours later. By this time the plane is pretty full and you should be in your final step. DON'T COMPLETE THE CHECK IN PROCESS!
About 3 or 4 hours before your flight do a final check to see that your seat is still the best one available. If you want to complete the check in process; or do so at the airport ... your seat is your seat.
No-frills Airlines and Seat Reservation Fees
Unless I am booking a premium seat I refuse to pay a fee to reserve my seat as I can do this via the online check-in process for free! A couple of airlines have gotten wise to this loophole and will give you an allocated seat with a paid option to change. In this case, if you're allocated a non-optimal seat - exit the check-in process and start again in 10/so minutes when you will be allocated a new seat as other passengers have checked in. You will also be able to pick up on the automated seat allocation process used by the airline by repeating the check-in process a couple of times.
I hope that you have found this useful and relevant. I have spent way too many hours in the sky, both in the pursuit of work and pleasure, and I use this myself. If you have any useful tips, please leave them in the comments section and I'll include them at the foot of this post.