This should be the model for the Democratic response. No need to talk about her lack of experience - that speaks for itself. (Of course it's fine to mention troopergate, her extremist positions, and the overstatement of her reformist credentials.)
Instead, make it about McCain's breathtaking lack of judgement. Keep hammering at the fact that McCain met her once, called her up at the Alaska state fair, and then put her a heartbeat away from the presidency.
This alludes to everything without saying it outright. Erratic decisions like this point to a deteriorating mental state and/or an intrinsic dangerous impulsiveness. You make people think of the hothead issue and the age issue, without saying them outright.
At the same time, you make people question her qualifications without the pitfalls of confronting them straight on. Her unpreparedness speaks for itself. But if you're focusing on how McCain made the decision - that he knew barely any more about her than we do - you run no risk of being called sexist. And you don't give the GOP surrogates an opening to make their ridiculous arguments about hypocrisy, top vs bottom of the ticket, or "executive experience." People will think of it all on their own.