Underway, stay in the fresh air on deck and out of the boat exhaust. It helps to stay in the center of the boat, which moves the least, and watch the horizon. Try to stay busy setting up your equipment so you'll be prepared to enter the water as soon as possible. Reading and intricate tasks tend to promote seasickness, so leave the needlepoint at home. 

If you do get sick, go to the leeward side (wind at your back) and have someone come with you (no joke - to hold on to you for safety when you lean over the side. Stay out of the head (that's about the worst place to go), and try to relax. To avoid seasickness, many scuba divers take seasickness medication if it might be a problem - check with your physician or pharmacist if you need a recommendation on the type that's best for you.