Working in a clothing store

Been working in a clothing store for a while now, but can't seem to get through the customers' "just looking" shield. Just looking is okay, of course ... I have no problem with people who come in whithout intentions of spending money. It's just that even though I say something completely random - just casually initiating a conversation or a simple question or whatever - a lot of people instantly turn on their shields, and do not at all want to have a conversation.

Thought I'd get your input on this. Does anyone have any tips or any useful experience from retail buisness (mainly clothes)?