How do you get a broken key out of a keyhole?

UPDATE:

Sorry for leaving you all in suspense. I managed to get the garage door open from the inside with a spare key - 2 days later! It was impossible to get the key out. Here's how I did it:

The door in question was the way into the garage from the back garden. The garage backs out into a lane where there is a traditional garage shutter which typicaly could only be unlocked from the inside. Knowing where this lock is at floor level I used a carjack to get whatever lift was possible on the shutter door - we're talking a couple of inches - and my younger sister(small hands) slid her hand under with the key and miraculously managed to open it with little or know room for manouevre. Then all that was left was to lift the shutter door open and open the garage door from the inside.

With the door now open I had to take the lock apart because the broken key couldn't be pushed from the inside. It's still stuck in the now defunct lock by the way! I have replaced it with a spare lock I had in the house.

Thanks for all your suggestions. Some of them were ingenious and I'm amazed they didn't work, but this key was just not budging.