What should I wear to an open interview/open call at a clothing store?

Since these are fashionable stores I would dress according to their style but slightly better.

You should wear clothes similiar to the ones they sell. Make sure that they are well co ordinated as that will demonstrate you have good taste and will be able to assist the customers with their choices.

Something cool and casual, yet still put together. Try some darker wash jeans, a nice solid colour v-neck t-shirt with a nice dress shirt over top and maybe some converse or high top b-boy shoes.