Okey dokey, folkey... I'm back.
Mary Gae's finger tug-of-war (from Seminar A tape; I hope I don't get in trouble for sharing it online): Link fingers together and pull. This strengthens the end joints. Without strong end joints, you don’t get good tone.
Mary Gae also works with dowels to build up the hand arch. This is explained on the Seminar A tapes.
Before I elaborate on the football, I want to mention one other thing I thought of. This may seem obvious, but always check that the student is sitting at the proper height. If the student is sitting too low, his/her wrist will drop & the hand will not be in proper position. Most of the time, correcting the seat height only involves putting a cushion or telephone book on top of the piano bench for the student to sit on.
Now, about the football: Buy one of those soft (but not too
soft) vinyl-covered Hedstrom footballs that are about 6.5" long x 4.5" wide. Have student place hand over football. The shape of the football helps student develop anatomically neutral hand shape.