When a phone goes missing, the real danger isn't what the device costs but what's inside it: banking apps, your messages, your photos, the open session of every account you have. The good news is that both Android and iPhone come with tools for remotely finding, locking and erasing a lost device.
The steps below are listed in order of importance — when panic hits, work through them from top to bottom. The last step, on the other hand, is the one preparation to make today, while your phone is still in your pocket: confirming now that these features are turned on. Once it's lost, there's no way to turn them on.