Here's what I recently read, but I never tried it myself.
Take the broken key end, put just a drop of super glue on the end, wait a couple of seconds, then push it into the lock, contacting the other half of the broken key. Hold it still for about 30 seconds--or whatever the superglue instructions tell you to do. Pull it out and hope the glue stuck.
Do not use an excessive amt of superglue--you will have the key glued in place to the tumblers.
If you no longer have the key half, cut the cotton swab off a Qtip, put the super glue on the shaft of the Qtip and try that.
I did get a broken key out of a Fiero door once, by straightening out a small gold fish hook, slightly knocking the barb down a bit, then push the straightened out hook into the keyhole with a pair of pliers. You want to push it in from the top part of the slot--not the bottom. The objective is to let the point go past the end of the broken key, and let the barb grab one of the recesses of the key on the tumbler side. You will probably have to push it in and pull it out multiple times before you 'hook' it.
Good luck--it's all hit or miss.
[This message has been edited by maryjane (edited 04-26-2008).]