Proper way is to remove the lock assembly and dismantle it, most locksmiths and garages will do this, some locksmiths will have tools that "might" tease it out..
If you can see the key bit down the lock then there is a method that sometimes works but you have to be extremely careful and patient.
Depending on the size of the hole you need a matchstick or cocktail stick - whichever fits down the hole without touching the sides.
Put the lock in the steering lock position so that the key could be removed if it was intact.
Cut the end of the matchstick cocktail stick ensuring you have a flat end.
Put a VERY small amount of superglue on the end of the stick.
Push the stick firmly onto the end of the broken key, making sure you dont get any glue on the sides of the key hole. Hold it still for at least 20 seconds and then try and withdraw the key.
Best place to push against is the thickest part of the key near the middle.
Needless to say I don't hold any responsibility if this goes wrong (glue in lock) but I have succeeded in doing this on more than one occasion. Reason for cutting the matchstick is that they are wax covered but sometimes its worth a first attempt without cutting it. After cutting it the wood is sometimes very absorbent and superglue dries almost instantly if it is absorbed.
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