I have used the salt method (using cheap table salt), the sink spray, and the sink dunk method to clean silk flowers. They all work. I prefer the salt method for larger arrangements, unless I want to mop the floor!
Hobby Lobby, Michael's, and I would guess JoAnn's sell a silk flower cleaning spray. It works, too. The dirt just fizzles away. Where does it go? I don't know, but it is gone. The chemicals are a bit of a concern to me.