A traditional cure is getting some carbon into the stomach. For humans that is easily done by eating a bit of burnt toast. I don't know what the doggy equivalent may be - maybe coat one side of said toast with pate or similar?
Also, in humans, seasickness is generally worst when you lose sight of the horizon as a reference point so if the same applies to dogs, get him a somewhere to sit where he can see out.
Two good ways I heard were, as Nick said, getting carbon into the stomach by eating burnt toast, alternatively if your dog likes lemonade, try giving him/her some of that, same affect. The other way is not very feasible, but if you put an earplug in one of your dog’s ears, it should help.
The only time my dog was seasick he fell overboard and had to be puled back ob, he has never been seasick since....I don't know if anything can be deduced from that, but it's the truth.
TôMö
So...
next weekend it's down the vet for him, then home alone, tied to a tree in the Solent, fed burnt toast and lemonade, dunked overboard and given a packet of ginger biscuits.
Just couldn't get this sorta information anywhere else
Now I come to think of it, wifey gets a bit seasick at times - any ideas how I get her to the vet?