My Dad just went to London for a couple of weeks, which, considering we live in Australia, is a long way. I myself went to Brazil last year via South Africa (the long way).
What we both do, is get in the seat, chill out, then treat the cabin crew very nicely (especially the purser), and then ask if there are any other seats available to change.
So when Dad went to London he got stuck between a bunch of screaming babies, so he asked the cabin crew if he could move, and shazam, upgraded seat
i was on the way back from brazil and didnt care about the group i was travelling with anymore (and i really didnt like the guy next to me) so i told the purser the guy next to me keeps falling asleep and moving in his sleep and hitting me =P she got worried so she had a quick look around and said the only available seat is in business (and on International Flights, business is those big booths where you can lie down, spread out, have some privacy etc etc. I played it up that i didnt want to get her in trouble and she said no worries
I was stoked
but usually, if you are nice to the cabin crew, and tell them the truth that the Bus dressed like a person sitting next to you is making you very uncomfortable, they will usually move you. its only if there are no empty seats that you will need to lie, but even then its not a certainty... in that case, take a sleeping pill, and hope they need to go to the toilet while you are asleep =P