When I worked for Honda, the parts manager, Yoshimura (yes, THAT Yoshi) said if any of the staff came to him for a new sump plug and the old one didn't have hammer scars on it, they would pay for it themselves. Whack the plug smartly with a light hammer several times. That reverses the stress on the threads and, with a little luck, breaks the bond. You can also use a drift on the flange to drive it round.
