Removing all NOT NULL constraints from multiple tables easily

I would like to remove all NOT NULL constraints from my tables.

I've tried ALTER TABLE, but I can't seem to get the syntax right, and the
docs aren't clear to me.
What is the correct syntax?  If there's another way, short of manually
recreating (dump, create, import) the tables, I'd love to hear it.

Thanks,

Dan Browning
Network & Database Administrator
Cyclone Computer Systems