Get up and keep moving around would be my advice. All kinds of travel sickness (sea sickness included) occur because your body knows you're sitting still but your senses tell you you're moving. That's why passengers in a car might get travel sick but the driver won't - the passengers are sat still watching the world pass by but the driver is constantly accelerating, braking, changing gear, steering etc.
Seriously, nine times out of ten the nausea starts because your body can't figure out if it's moving or not. I used to suffer massively from travel sickness when I was a lot younger, so I know it sucks, but just try walking around for half an hour or so. That always did the trick for me.