Well, to keep from passing out I would ask one of the flight attendants to change seats, provided there are any.

I experienced this with smelly, sweating, obese seatmates whose excess tissue (fat) took up over half of my seat. Torture.

His/her body ballooned over and under the armrest. I spent 5-1/2 hours painfully perched on one hip, jammed against the window.

Every seat was full on the airplane.

Speak to the stewardess about it. Otherwise not much you can do, after all how many planes do have showers and washing machines.

Break out the perfume!

Step outside for some fresh air.

Kindly stand up, grab my deodorant out of my carry on (because fuck paying for check in fees) and quietly hand it to him and say "please do the right thing and I'll buy you a drink"

Having been in this situation I asked the hostess to change my seat. Since there were no other seats available i complained that he should not have been allowed to board the plane. My ticket was refunded in full and I still had my return flight ticket.