Dancewear without rhinestones or decorations: dress without any decoration can be washed by machine with temperature below 30 degrees. Please aviod mixed-washing it with your own white or light colour clothes. Some of the colors might come off and diluted with your other clothing. Better to wash it by hands with mild soap in cold water to keep it in good condition.
Competition costumes with decorations - rhinestones/sequins/beads/feathers: Only spot-clean by brush when necessary, and NEVER WASH them directly with water in order to keep the best condition of the dress.