One common method in the garb circles is to get some relatively small ribbon, and to sew that into the hem, so you can gather the cuffs and, in this case, waist, with that and then just tie a bow in the ribbon to hold it gathered. Effective, and a nice little decorative element too.

Collars are sometimes just gathered around a loop of ribbon, and sewn shut so they are permanently gathered. And once something like that is gathered, it tends to completely hide any unevenness in the hemming.