Seasickness is purely an issue of mind over matter. (Here I resist the urge to insert the usual crack that if you don't mind being sick, then the fact that you are sick does not matter.)
Seriously, the best way to prevent it is simply to know that you are not going to get sick, regardless of how bad the weather and the sea might get. Once aboard ship, don't even entertain the thought that you could possibly ever get sick. If, however, you do find yourself feeling slightly queezy, either go out on deck in a place where you can get the wind in your face and look out at the horizon or go to bed until it passes. Additionally, do keep food that is not unsettleing in your stomach -- and a glass or two of wine (not beer, and not hard liquor) is great for settling the stomach!!
BTW, there are several other conditions that may cause you to be sick on the first day or two at sea that are not real seasickness. A lot of people mix a combination of being overtired as a result of being overextended for several weeks or months prior to the cruise with a tad of jet lag and a lot of sleep deprivation, then wonder why they get sick. You can tell real seasickness by the fact that it has two stages. In the first stage, the person will be afraid that he or she is going to die. In the second stage, the person will be afraid that he or she is NOT going to die. Very few people ever experience this.