I never dress up just because I'm going in an expensive store...I feel your attitude and how you carry yourself makes a greater impression than what you're wearing. If you walk in with sweats and runners yet show the sales associates you know what you want and what you're talking about, there is no problem.
I can't be bothered to dress up if I don't really have to. This way the SAs who don't snub me gets the business - they deserve it for not having double standards.
I do this all the time to "test" the SA's sincerity and professionalism. It's my money, I get to decide who earns it