If you use Chrome, check out the "How your ads are personalized" page. It gives you a glimpse of how accurate Google's targeting is based on all the data they've collected.

I went to a great Google Fireside Chat with Michal Kosinski on data privacy. He said that the only way to get around big data is to spread misinformation. Chose the opposite of your tastes, search for things you don't like, etc. https://www.youtube.com/watch?v=VUwBcTgzbtU