Don't look like you're trying too hard. It's part of our corporate culture that you don't have to dress up to be serious, but the groups I support are sales and operations, and they all dress in a business casual. They wear nice slacks or khakis, nice shoes (lots of Eccos, actually, now that I've started paying more attention) but they wear quality clothing. So I choose to dress like them and buy nice stuff. I try and invest in nice shoes and belts, and also nice quality shirts. You can get away with buying inexpensive pants -- Dockers, for example.
Here is the rest of Jason's advice. Does anyone know if these tips will help you get past the first interview with Google?