How do you stop the hitting?

It's a serious parenting question.
I want to know how to stop my child from hitting me, Kevin and the dog. Thomas has become increasingly violent, and I don't know how to stop it. He routinely throws things, punches, hits us with objects and with his fists. He throws verbal barbs like a bitter old man who's mad at the world and everything in it.

How do I stop this cycle of physical outbursts?

He's not always angry and violent. He can be very sweet and loving... but man, oh man... when he's not? Watch out, world.

Please. I'm VERY open to suggestions.