Watts has the best answer for sure. A trick I use is to take a piece of piano wire or guitar string, crimp the end into a sharp curve and use a grinder of some sort to cut it off so that it has a very sharp edge.
Spray the lock cyl with a good teflon based lubricant and wait a few minutes. The idea is to stick the wire into the flat surface of the key and the sharp edge sinks or grabs into the key and pulls out the broken piece. It works.....sounds mickey mouse, but I've done it many dozens of times. Once you get the broken piece out, take it and the other piece of the key to a locksmith and have them cut you the right key. Good luck!
Phil
------------------
87 FIERO GT 2.8 5spd
0-60 in 6.8 seconds!