More and more airlines allow you to pick your own seats on their website before take-off. If you're lucky you get to see how the seats are mapped so at least you know which seats are window-side and which are isle-side.

However, what you don't get to find out is if you'll end up folding your legs on your lap for the entire trip because there is no leg space. Or that you won't be able to hear anyone during and also, for the first few days after your flight, because you were unfortunate to have a seat next to the engines. Or maybe you might have picked another seat if you'd known the tv would have been right above your head where everyone could see it except you?

This is where website seatguru.com comes in to the rescue! I'm on holiday as we speak. My airline company also allowed me to pick my own seat.
So first, I went to seatguru.com, picked the correct type of aircraft, which gave me a map and detailed information per seat for that type of aircraft.

I avoided the noisy seats close to the engine or close to where cry-babies were likely to be changed. Also no chilly seats for me, so my seat had to be far enough from the emergency exits (but ofcourse not too far). Ah and interesting.. some seats in the front had AC power outlets. So I ended up picking my perfect AC powered seat, not too cold, not too noisy, with some great help from seatguru.com.

Another tip to remember:
Usually you get to pick or change your seat up till hours before take-off. If you spot any triple seats, where only one spot on either end is taken, take the seat on the other end. It is very unlikely someone would pick that last seat in the middle if your flight isn't fully booked. That means no elbow fighting and both armrests all to yourself for the entire flight!