[Fey]

Best advice for a coworker like that, be super, ridiculously polite to her. She's being rude, don't sink to her level. Every time she insults you just smile and say 'Thanks for your opinion, I value all the thought you put into it!' When she's rude to customers walk over and be super polite to them with something like, 'Hi! You know, I had to come tell you that I love your hair(or something else non-clothing related). I'm sure [insert bitches name here] is taking great care of you, but don't hesitate to ask if you end up with questions!'

This is going to do a couple things. One, the customers are going to see you in a positive light, and by comparison she'll look incompetent. This WILL get back to management. Then management will see that you're being very positive/polite in the face of all her scathing comments and see you in an even BETTER light. She'll end up hurting herself in the long run. And who knows, after a while she'll either get reprimanded enough, or confused enough, to stop all the rude behaviour.