The only way you can realistically get those types of photos is if you use a concealed camera hidden in a shirt pocket or one of those new android phones with a built-in 6MP camera where you can pretend to be taking/making a call while you're shooting the picture.
I wouldn't attempt to take photos like that with a traditional camera because if caught, your camera could be confiscated and you would be outed as a shopper. I don't accept shops that require covert photos because to me that is simply industrial espionage and I don't get paid near enough money to be taking those kinds of risks.
I get much better shots when I am not rushing to take a photo and nothing is worse then getting home only to find the pictures you've taken are too blurry to make out the images or you didn't get the entire view of the target you were aiming at.
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One buzzard to another while circling high overhead (paraphrased), "Patience hell! I want to shop somewhere."