An Extra Set of Keys
It seems like an obvious suggestion, but always be sure to have at least one extra set of keys. I have heard many stories lately from friends who have been locked out of their own homes.
Each adult in your home, and any child who is mature enough for the responsibility, should have their own house key(s). It also makes sense for everyone to have a set of car keys (for their own car, and for those of other household members). You should also have at least one spare set of keys. Leave them with a trusted neighbor, friend or relative, or even leave a set at work in your desk. If you insist on 'hiding' a key outside your house, consult a security expert on how to do so safely. The old trick of hiding one under a rock, or in a planter, has been used so many times that burglars have caught on!
You never know when you might lose your key(s), or lock them inside your house or car. For your own safety and sanity, make sure you have extras!