How can we prevent this from happening again?

Prevention is better than cure! To avoid reliving this scenario, a few simple actions can make all the difference:

Lubricate the lock regularly with a suitable spray (avoid classic oil which can clog the mechanism).
Check the condition of your keys : a worn or bent key is more likely to break.
Protect your locks from temperature variations which can affect their proper functioning.
Proper maintenance of your locks can help you avoid many problems. And if your key breaks again , you will now have the right reflexes to recover it without stress !