Sometimes you just don't know your own strength. Keys are susceptible to breaking in locks and while you can replace a key, you still face the question of how to remove the broken piece from the lock. Removing your broken key may require one, or several different tools, depending on both the key and the condition of the lock. Older locks, for example, may require lubrication before the key will budge.
Spray lubricant into the keyhole to help the key slide out. Press the nozzle of a can of spray oil up against the lock and squirt it in.
Reach into the lock, with specially designed key removal tools. These tools are used by locksmiths, but are also available for personal use. They typically come in kits of various sizes, so whatever size your lock, reach in and latch onto the key's grooves with the tool's barbed end.
Use long and thin gripping tools, like needle-nose pliers or tweezers, to reach in and grip the key to pull it out. Roughen up the inside of your tweezers near the tip -- this gives it a better grip.
Stick a coping saw blade into the lock teeth-first, using the teeth to hook onto the key's grooves so you can pull it straight out.