I don't know if I can really be specific about it, different styles look good on different girls. If being approachable is what you're going for, I don't know that clothing is a big deal for guys. Just wear something that makes you feel confident. If you want guys to approach you, look around the bar and see who is looking at you. Smile at them. That's a much more obvious sign that you're available than what fashion choices you've made. "I'm wearing a necklace, come get me big boy" isn't exactly the kind of thing a guy is going to pick up on.
General rule though, don't dress slutty unless you're looking for a random hookup. A little skin is good, but a girl that's wearing a mini-skirt AND a belly shirt or something along those lines is basically just saying "I'm a whore. Come have a one timer and pick up some pube lice shampoo on your way home."