Sales staff who judge people based on the way they dress are pretty idiotic. I don't dress up when I go clothes shopping, in fact, I do the opposite. I try to wear a t shirt and jeans, so if I need to try something on, it'd be quick. If that means you don't think I am willing to buy, or even pay full price on items, then you really need additional training.
Some store just have better services. Bergdorf Goodman, for example, never makes me feel like they're too good for my patronage.
I've worked in inside sales in industry, a far cry from clothing retail, I'll admit. No matter who called, I treated them with respect and kindness and they knew I would work get them the best price (I'm in a price based, not quality based industry). That being said, I had to be smart about it. Our best account, they get my first attention. If you literally only buy $100 worth of product from us once a quarter, you have to wait for pricing/quotes, etc. Most customers understand that too. That probably holds in retail, if you know a certain guy will buy, its not beyond the pale to help them more than someone who just appears to be browsing.
Only when a customer is rude ("you do this for me now!" or "Meet this price or else!") would I tune a customer out. You don't be rude when you do it, you still maintain the kind demeanor, but they get lowest priority because I'm a human being, not a dog doing tricks for your business.
If you're shopping and you are perfectly kind, there is no reason for a store to be anything but very helpful and kind, end of story, anything else is unacceptable.
+1 I couldn't agree more. It has to be a very elitist store to turn away someones money based on age, skin color or appearance.