Summary: You register at www.StopPoliticalCalls.org
, and you are put on a list. Some campaigns have agreed not to call numbers on that list.
My initial research suggests that it is a real organization, although I'm not sure how effective it is. I used a Mailinator address so that I did not have to give out my real email address.
You have to register twice if there are two voters in your household.
BTW, automated calls are illegal in California, but the law apparently is not enforced.