This is an easy one, book your seat on the window side.
If you forgot to set a reminder to book your seat on time and all window seats are booked already, the worst one is of course the seat in the middle.
INSIDER TIP: If you travel with someone else, let's say as a couple.
In this case, you should book the window seat and the alley seat!
If you are lucky, no one will book the seat in between, and you have a FREE THIRD SEAT.
If it will be booked, the other person will most likely want to change his less valuable middle seat with the one on the alley.
Maximize this tip by booking your seat in the back of the plane, because people will tend to book seats in the front of the plane.
On many emergency exits you will have the most legroom.
But be aware that a couple of window seats on the emergency exit row have bulky doors which will protrude into leg room space.
Take a look into Seat Guru about seats which do not have other seats in front of them.
But don't book the seat right behind the Galley.
Yes, you have more legroom, but you can't stretch your legs because the wall of the Galley leaves not enough space.
And it's more comfortable to be able to stretch your legs fully on a normal seat, compared to have a little more legroom, without being able to stretch your legs.
Near to the Exit
(Only for flights where you have to show your passport at the destinations airport)
I'm not talking about emergency exit, this is a non factor because the chance you will use the emergency exit is extremely slim. I'm talking about the exit where you will board the plane most of the time this will be the front.
This is very important on flights, where you afterwards will have to entry the country and show your passport. On all other flights, to be near the exit isn't that crucial.
On flights, like from Europe to the USA, (unless you are an American citizen) you will have to stand in a line to entry the country, where you will be closely looked at, by a police officer on a counter.
You will have to give your fingerprints, they will ask strange questions until you can leave the airport. The line is so big that it can last easily one hour until you reach the entry-counter. So imagine a trans Atlantic flight for 9h, finally arrived, got out of the plane, and then you have to wait for another hour in a line. Horrible! And to make things worse, it's prohibited to use your phone.
So, the faster you can leave the plane, fewer people stand in line because they are still leaving the plane and you waste less time in the line to the counter.
This will save you 30min to 1h.