This is something I just can't bring myself to do. I disabled it years ago, along with all the other personalization options.
I don't mind giving them my location. I don't even mind them analyzing my mail. But I search for all kinds of crap, and I really really don't want anyone building a profile on me based on that.
The manual customization options in Google Now are sadly lacking. I've been able to add a few sports teams, but that's about it.
I suppose I could switch all my default searches to DuckDuckGo, then only use Google judiciously to deliberately train it. Hm, I think I will.