In that situation, I answer with "Not yet thank you" in a confident manner. I do not need to justify or explain my presence in a store to the employees. I'm a potential customer and dress the part, and therefore should be treated as one. Living in Germany teaches one to be polite but firm.
I don't really think its necessary to dress the part. You are there to buy clothes not to impress sales people with your fashion sense. As previously stated maybe it was because I've been really really lucky in the past and have not run into a snobby sales person as of yet but usually when i go to browse the high end stores I'm dressed like crap, i.e north face jacket, t shirt jeans and sneakers, and in the summer time I'm in shorts a undershirt and flip flops.