There are a ton of discussions about this, many in the Dive Medicine section.
A few of the basics:
If you are using pills start taking them the night before you dive to get the medication in your system.
Don't eat greasy foods, acidic foods/juices.
Don't eat anything that might have an unpleasant texture when it comes back up (nuts and carrots come to mind).
Have a nice bland breakfast (dry toast, things like that) and plenty of water to start your day, don't go on the boat on an empty stomach.
I eat some saltines before I get on the boat, immediately after I get on the boat, as soon as the boat starts moving and any time my stomach even THINKS about feeling queasy. Ginger snaps, candied ginger and peppermints are also used by people to keep their stomachs thinking of something other than being ill.
Make sure to stay hydrated.
BTW, you probably know dry heaves feel worse than actually vomiting. When you are seasick you're going to go through the heaving motion, eating some saltines and drinking some water or Gatorade as soon as you finish tossing the last batch takes the misery level down a notch.
Ber :lilbunny: