I always have my View Catcher with me when I’m out sketching. It’s a simple device but can be a big help when trying to work out an interesting composition. I bought mine at Ken Bromley Art Supplies in the UK. Visit here if you would like to order one yourself.
This video is a very long-winded demo of the View Catcher but it does give you an idea of how it can help you with your urban sketching.