I imagine the sales staff are just doing what they are told. They are probably under instructions to approach everyone and to hover around you at all costs, usually this involves pointing out inane facts about the products, like "this one is black"... Bauhaus springs to mind. I usually smile and ask politely, "can you leave me to look around and I will call you if I need any help". It probably works 50% of the time.
Wearing headphones works 99% of the time but you feel like a bit of a jerk for just ignoring them. I find it hard to do that.
Anyone have a better strategy?