It may seem like a good idea to leave a spare key hidden under a flower pot or doormat in case you get locked out of your house. But that's an open invitation for a burglar to walk inside without any difficulty. Someone could also see you retrieve the key at some point, giving away your hiding place.
Instead, give a spare to a neighbor you know well or friend who lives nearby for safekeeping. Since most people now own cell phones, if you lock yourself out you can call for help or walk over to the person's house. You could also put the spare into a combination lockbox and hide that somewhere outside.
Remember to never put any identifying information on your house keys. If you lose them, and someone else finds them, it would be fairly easy to trace them back to your home and break in.