Step one: use the new key.

Since that didn't work for me, I sawed mine off and banged it against a table a few times, then it worked once (took another several tries to make it work again). Too bad it was after I had sawed it off. Try a hammer or something (not on the key, on the ignition housing from behind).

If you do get it working I would take it apart (very easy when the key actually turns) and see if the tumblers are worn. You just stick the key in the cylinder after you've removed and see if any of the tumblers are still sticking out the sides. If they are I'd suggest getting a new tumbler.