I/O has a section on wrist flexibility.
Also try these:
YouTube - Closed Chain Wrist Mobility Drill - Nutrex Solutions
YouTube - Active Wrist Warm Up - Wrist Mobility - Nutrex Solutions
(on the second one I try clutching my fingers and getting a mild stretch)
Also:
Grip with less fingers. I grip with only the tips of the fingers and the pinkie only lightly touching the bar.
Elbows closer, fingers wider - makes it easier untill you stretch.
Grip a medium size BB with the clean grip and try pushing your elbows up to get a stretch.