Hate the look on people's faces when yo say "Excuse me!", to have a seat. Instead of moving in, they would reluctantly (read taking all the time and blocking inward traffic) move out and let you take the middle seat..
Being an expert in the daily commute, nowadays I can easily spot the right seat. The technique is very simple - see if the persons occupying the window and aisle are talking to each other..If so, they know each other and are just spacing themselves out. If you ask them to move in, they will have to budge- no choice, and you get the aisle !! Works all the time :)
