* Avoid eye contact.
* Lean against the window and stretch out your legs.
* Sit on the aisle seat and listen to music to pretend not to hear people asking for the window seat.
* Place a large bag or multiple items in the empty seat to make it time-consuming to move.
* Look out the window with a blank stare to appear crazy.
* Pretend to be asleep.
* Put your coat on the seat to make it appear already taken.
* If all else fails, lie: Say the seat has been taken by someone else.

Source:
http://www.wired.com/wiredscience/2012/08/bus-strangers-seats/