Murray already wrote most of what I'd have to suggest for out-and-about shooting - essentially, be prepared to accept what the environment you're in has to offer. After your mental/expectation slate is clean, get a sense of the overall, the mood that it creates. Then, start narrowing your view in increments, down to the little macro-level things. That way, you can be receptive to images at various levels as you wander through the environment.
There have also been some interesting discussions on "vision" here that might help you with your overall approach. A search of the archives should turn them up.