FDA-approved acupressure wristbands and various over-the-counter medications and natural treatments can help prevent seasickness, and should be used before you start to feel ill. Consult your physician before choosing to use either.
Your location onboard and in proximity to the water can also help. The lower you are to the water line, and the closer you are to the center of the ship, the less movement you will experience. When looking out at the water, keep your sight on the horizon line.