All doors that lead to the outside should be metal or solid-core, 1-3/4 inch hardwood. Most hollow doors can be easily broken through.
Each door should fit in its frame with no more than 1/8-inch clearance between the door and frame. A metal lining on the inside of an exterior door can prevent drilling, sawing, or kicking through.
Door hinges should always be on the inside and designed so that hinge pin cannot be removed from the outside.
Secure sliding glass doors with commercially available bars or locks, or put a wooden dowel or broom handle in the door track. Burglars look for sliding glass doors because they are the easiest to open.
Locks
Make sure all doors to the outside have good locks -- deadbolt locks with a minimum 1-1/2 inch bolt.
Make sure locks are also installed on screen and storm doors, garage doors, cellar doors, patio doors, and any other door that leads to the outside (including second-floor patios or decks).
Always use the locks you have, on both your home and your garage. Lock up every time you go out, even if it's only for a few minutes. Almost 50% of burglars enter homes or property through unlocked doors or windows.
Locks on doors should be placed at least 40 inches away from windows, glass panels, and other potential openings such as mail slots. Make it hard for a burglar to reach in and unlock your door. You can also install double cylinder and deadbolt locks that need to be opened with a key from the inside as well as the outside.
Keys
Never hide keys outside, such as under a bush or in a flowerpot. Burglars know where to find "secret" hiding places. It's much better to leave a key with a trusted neighbor.
Don't place identification tags on your keys or key rings; if you lose them, you give potential burglars help.