Excellent tips in an article on an excellent mobile app I use: JotNot Scanner Pro (all of $.99)
Note: I couldn't get more than a white page with this link using the latest Safari on my office Mac, but it works just fine in the Chrome browser. Bizarre. I use this app on my iPod touch and it works very well. (I've used it for things like photographing a quote off the screen of my iPad or a book or magazine, and sending that pic directly to my Evernote account (in the cloud). It's great that it easily corrects for pages or whiteboards so they aren't under-exposed to a middle gray. :-)