I have the same "cupping" problem with Arista 400. Hanging it with weights overnight has no effect, I've got two large plates of glass I've sandwiched the film between for a couple of days that helps some, but its still not as flat as the film I get back from the lab. After a week or two of being smooshed between the glass (or on the bottom of my negative binder) they are pretty flat. I might try the ironing trick, I've got a bunch of shirts that need to be ironed anyway

Adam