In my experience, I've just cut up old, thin, white towels, or old white T-Shirts, or old, thin white washclothes, and putting one in each load to attract any loose dye from noncolorfast items works best. You could also buy cheap terrycloth and cut that up into squares about 8" x 8".
In order to make a homemade dye magnet you should take one teaspoon full of Epsom salt and almost one litter of water and then you should add about 50ml of Vinegar to this then stir it clearly and immerse the dress for a few hours and then wash in cold water.