Here are all the things I've found useful or inspirational...
Creative Review has posted up a series of images showing prominent streets of London by Austrian artist Gregor Graf. He basically photoshops out all of the usual street clutter - people, cars, shop signs and branding - leaving you to guess where the street actually is. I'm more amazed that he's captured London with blue skies and fluffy white clouds, or is that just more photoshoppotrickery?
Before the advent of the computer and digital retouching, photographs were manipulated by hand. Here's a great round-up of some of the most infamous manipulations old and new. There are a fair few magazine examples in there too.