Where to keep a spare key?

You see it all the time on television and in the movies. People hide a spare key to their apartment in a light fixture above the door or a potted plant in the hallway. The idea is if you lose yours or need a friend to get something at your apartment, there's the key.
What this does is leave you completely at the mercy of anyone who takes a minute to look in these areas. Burglars are the exact type of people that will take the minute to look. Doing this is no different than leaving your door unlocked.
If you want to keep a spare key somewhere, leave one at a friend's house. If you have a safety deposit box you can even put one in there, but never leave it outside your door.
If a burglar manages to pick or break the lock on your door and get in the apartment, an alarm system will often scare them away. They won't know the code to enter in the control panel and they won't know your password to give the monitoring company.
The easiest thing you can do is make it hard on criminals.