I took an entrepreneurship course a while back that would have us do daily "opportunity reports". These reports were a list of 50 business ideas. They could -- and most were -- awful and not feasible ideas, but the point is to produce 50 ideas every day. After the course of a few months, I noticed it started changing the way I see the world. Everything that I came across (blogs, standing in line at the grocery store, etc) I would be looking for opportunities.

I think that's a good place to start.