One of the smartest things you can do on a regular basis to deter burglars or other would-be bad guys is to make sure your doors and windows stay shut and locked. Obviously, if you live in a city, where the population density is higher, you have a greater chance of having home break-ins, but some small towns and even the countryside get their share of criminals and kids looking for money, drugs or things they can sell. In some places, people just make sure their doors and windows are locked before they go to bed: in others, it's wiser to have permanent iron bars installed outside ground floor windows and to have alarms on windows and doors.
 

If you like your fresh air, you can add tight, well-fitted window screens to your frames, but if someone really wants to get into your house, any pocket knife can cut through a screen, so make sure to get the screens that can be raised slightly and locked into position. Storm doors with locking screens can also help get the fresh air in and keep unwanted visitors out.