figure out what the minimum salary you would need to work for them. interview at other companies. get other job offers. tell them that you want to work for them, but you have other offers elsewhere and the minimum you'd accept is $xyz.
you must be prepared to walk out if you're going to negotiate. if they balk, tell them they have a week to reconsider or you'll accept a position elsewhere.
also, don't forget that google does have a lot of intangibles and perks that other companies don't. in addition, working at google is CV gold. they're probably trying to lowball people and get the people who just want to work for them in their offices, posisbly weeding out the people in it for the money.
Is it really? I mean, is there any evidence that Google alumni are better paid when they leave? There are an awful lot of companies that look "good" on a CV. Goldman Sachs looks pretty good on a CV, but they don't play silly games with lowballing new hires...