You can add a contrain on the dress around where the belt is which will make the cloth behave as if there was a belt holding it tight to the body at that area. And also contrain the belt if you havent already-

My guess is that would give you the best result. 

Otherwise you could try to soft rigid the belt, which will make it follow the dress, but think that might end in unwanted result as the belt would appear to be very loose and unnatural. :)

If the constrain doesnt work try to post an image of the belt as well. Maybe before you have simulated the cloth.