I needed to update a column with the date and time whenever a record changed in my database. So here is the recipe for the trigger in MySQL 5.0.
DELIMITER $$
CREATE
/*[DEFINER = { user | CURRENT_USER }]*/
TRIGGER `my_database`.`UpdatedOn` BEFORE UPDATE
ON `my_database`.`my_table`
FOR EACH ROW BEGIN
set NEW.update_on = now();
END$$
DELIMITER ;
