I was never seasick until the last couple of years, started taking medicine for acid reflux and my tummy aint what it used to be!
I do alot of the things mentioned above, avoid the greasy breakfast, watch the horizon, etc... I have found that go old peptobismol works pretty good as a preventative. I take the caplettes cause I really don't like the taste of the liquid or chewables. I also try to do as much as possible to keep busy. I will set all the downriggers, pull all the gear when there is a fish on, wash the deck, gut the fish, play with the gps or finder, anything to keep busy. I also make sure to stay up wind of anyone who is seasick, nothing like a little wiff to get you started.
Last of all, have a thick hide. Your buddies will raz ya, the other people on the boat you may not know will treat you like you have SARS, the bait boy will giggle and the captain will roll his eyes and shake his head, but WHO CARES! To HE double hockey sticks with all of 'em. Have fun, puke if ya need to and make sure you track every mess up your buddies make for a year so you have some ammo when the feathers fly!