If this happens a lot, just create a story in your head that you can use each time, so you are ready with it.  No need to give strangers personal information.  Or buy a ipod or other music device.  Even if it isn't on, have the speakers in your ears and people probably won't start a conversation. 

Hi, I’ve noticed that the situation you’re describing happens very rarely to me. I just came back from a trip where it was obvious that my seatmate would have liked to talk, and probably because I read your post I enjoyed analysing how I repel this kind of attempts (needless to say that he did not succeed to get a word from me, not even a hello or goodbye, which I agree is on the limit of impoliteness).

So the secret is body language. No need to make up stories or to use your ipod (unless you really want to hear music of course).

Here are a few pointers:

1. Start by being totally absorbed by in yourself. This could be done by reading anything, listening or even just gazing emptily into space. People usually don’t dare to force this kind of barrier, but if they do (a VERY noisy seatmate) you just look back absentmindedly as if you didn’t hear the question, mumble yes or no, and go back to what you were doing (or not doing).
2. Avoid eye contact at any price! Very easy to do as you just need to keep looking in the opposite direction. If by chance your gaze wandered and met his for a second, revert immediately your eyes to the opposite direction.
3. Pretend to sleep even if you don’t.

You sound like a nice and social person (I assume that as you wouldn’t be facing this problem otherwise),  so of course this is not a very social or sympathetic behaviour and might go against your nature but it is very efficient and you can use it sometimes as the mood would suggest to you.

Dear Kara,

Thanks, I'll have to give that a try. Yes, you're certainly right -- I am social, but maybe the right way to describe it is that I find almost everyone interesting in theory and very few people interesting in actuality. (The story I posted here: post is, as much as I grumble to admit it, pretty typical of me.)

But I'll try for special occasions to brush up on my haughtiness. Your descriptions are very convincing.