Wired lets us know about a sociologist who recently spent 3 years traveling around on a bus talking to strangers about how to not have to talk to strangers. Despite the incongruous data retrieval process (the people talking to her are obviously not good at not talking to strangers), the tips are fairly obvious and straightforward.
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.