I've heard some people use those vacuum packing bags. When DD was in full princess mode, I just put them on top after I packed everything else, with the bottom folded under to fit the suitcase with a sheet of white tissue paper under and over them (and between them if there was more than one which there usually was) and then I shoved the suitcase lid down on them. At the resort, I just hung them up in the bathroom and turned the shower on hot to steam any wrinkles out. We usually had the
Disney Store or Parks versions and those really did not wrinkled that much even is they were crammed into the suitcase. Other people may have better ways to pack them, but that worked for us.