The best way to approach this situation is to do so by asking your girlfriend for her help with your problem — and make it your problem. If you start blaming her for the way she dresses or putting her on the defensive, you’re going to get no where. Approach the situation as if you’re the one with the problem, not her, and you’d like her help in making you feel more comfortable. Try to leave any judgment about the way she dresses, out of the conversation. If you accuse her of dressing too provocatively, she’s going to get defensive, angry and lash out.
Tell her that you feel really jealous when she’s dressed to kill, and other guys are noticing and making comments. Tell her that you don’t feel like she belongs to you when she dresses like that when she goes out without you. Then see what she says.
Remember to tell her how beautiful she is, and that you don’t like sharing her with other guys — even if all they’re getting is an eyeful.
Let me know how it goes. 🙂