First, lay the dress out, with the front down, on a large flat surface such as a bed so that it is completly flat with no wrinkles and the centerline of the back is directly on top of the centerline of the front.. Pull the sides of the skirt out completely so that the skirt lays flat. Next, fold one side of the skirt over the center so that the fold forms a straight line with the upper body, then fold the other side in the same way. (If the first side extends over the second, the tip will fold in at the same time.) If the tip of the second side extends beyond the first fold, fold the tip over the center.
If the dress has sleeves, fold the sleeves over the back, also even with the side of the body. If the sleeves are long, fold again at 45 degrees to extend down the back and not overlap.
You should now have a garment of approximately uniform width. Fold the bottom up to the waste, then fold the body over the skirt.
Folded in this way, the dress should lie flat in your suitcase. It also should be free of wrinkles when you unpack it. If it develops creases along any of your folds, a quick "touch-up" with an iron will remove them.