I just want to add that when nausea is caused by a medication, it's best to find out what to use for it from the doctor or even a pharmacist, since some products can impede the absorption of the medication by the stomach or even neutralize it, so you want to be sure that whatever you use to combat the nausea doesn't negate the medication in any way.
Jaybay's suggestion to break up the meals into a few feedings a day is a good one. That way the stomach doesn't get overworked from big meals and isn't prone to being totally empty for long periods of time, which can allow acid to build up and cause MORE nausea.
If your dog is new to the phenobarbital it's just a matter of time before her system adjusts to it and the nausea stops, but the tips that Jaybay has given you should help to minimize that nausea until that happens.
Ghilly
It's possible the medication is irritating your dog's stomach, so ring up your vet and see if they can recommend an over-the-counter nausea med like pepcid. You'll need to get proper dosing information on any OTC med from your vet.
Another thing to consider is how your dog eats. If she bolts down dry kibble as fast as possible and then drinks a lot of water, the food blows up in her stomach and can only come back up. A higher quality kibble mixed with a bit of wet food can help with that. It can also help to split her meals up into 3 or even 4 servings a day - just be careful to cut down on the amounts of each serving.