I always try a two-pronged attack:

1. I call the most senior steward(ess) over and say that the passenger next to me is clearly not well, and I think it would be better for THEM if they had some extra space to stretch out in.

If that doesn't get me moved, I say:

2. That I am extremely under the weather myself, and worried that I may catch something, is it possible for me to have some paracetamol (or any other kind of medicine you know they stock on the plane) IF they are unable to move me.

That virtually always does the trick, I assure you. The trouble to the steward(ess) in getting you some medicine and filling out all the forms and getting a colleague to so-sign the forms and getting your address etc etc. is alot more than moving you (even if it means an upgrade!).

[I use the same argument for my other pet hate in the sky - being sat next to 'unaccompanied minors'!! "I can see the sky..." "very nice." "What does this button do?" "I don't know" light-on.... light-off... light-on... light-off... seat-reclined... seat-vertical... seat-reclined... seat-vertical... "Have we taken off yet?" "NO!".
Don't even get me talking about why they give children chocolate bars on planes. You know the little darlings will drop the bar in your lap at some point during the flight]
MF