The guy I work with in the shop just made a couple big (about 20"x24" and about an inch and a half thick) cutting boards and used alternating walnut and ash for contrast. He finished them and leaned them up against each other for a few days and when he laid them on a table each had a cup of a little over an eighth inch. He could afford the time and just set them aside three days ago with the convex side up. They are just about flat now.

I had the same thing happen with a wide but short ash board several years ago except it cupped about a quarter inch. It must really have been wet because it took flipping it about three times when the cup reversed itself. Works OK as long as you have the time to wait on it.