Have interesting opinions on Google apps — it’ll make you a much more memorable candidate. Try out some lesser-known apps (Google Flights and Trips are among my favorites, for instance). The same goes for other companies you might interview at.
To really impress at the Google APM interviews, you must be able to talk intelligently about common technologies such as machine learning, the cloud, and big data. You also have to know Google’s products, strategies, and competitors. You can’t learn this by just drilling interview questions.