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.
Actually, having a bunch of categories is *not* what you should do. Categories are meant to be a description of what your business is, not a place for you to stuff keywords for which you want good search results. This is spamming our algorithm, and even if it works temporarily, over time our algorithms will get smarter.
Fraser's, what you want to do is concentrate on generating information about you on the web. Encourage your customers to write reviews about your business. Basically, our goal is to know how well perceived your business is, and therefore to give our users a realistic view of the world.
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.