My wife had a problem with her feet cramping almost every night after work. She would also get them when we would snowboard, mainly when she was bucking in after we'd get off the lift.
I bought her one of these a few months ago:
Amazon.com: Medi-Dyne ProStretch Unilateral Stretching System: Health & Personal Care
She uses it every night to stretch out her feet. You can really sink your foot down and flex it better than any other options.
That combined with increasing the amount of water she drinks and adding a banana in her lunch have pretty much removed all foot cramping.