A Google Places Help thread has a Google Maps Product Manager, Jeremy Sussman, responding to how to help improve your Google Maps rankings.

He offered specific advice to one Google Places holder telling him not to take advice of someone in the thread telling him to add himself to more categories. You can, but only if it is relevant to the business. Let me quote Jeremy.

Jeremy also posted a how local search ranking works video, which I will embed here.

Do you think you should add your business to more categories and not listen to Google’s advice?

Forum discussion at Google Places Help.