Summarize:

Burglars can sometimes remove or damage lock cylinders by hammering, wrenching, or prying. Protect these with metal guard plates or protective rings on both sides of the door. Install guard plates with round-head carriage bolts to prevent them from being unscrewed. Free-spinning rings around the cylinders will prevent the use of a pipe wrench to twist out the cylinder. Many locks come with these already, but if yours doesn't, you can purchase them. The strike plate is the metal plate that surrounds the lock-set (the hole in the door frame where the lock bolt enters). All exterior doors should have heavy-duty metal security strike plates secured by four 3-inch screws. Many homes are built with lower quality strike plates or have strike plates that are secured with short screws that just attach to the door jamb, not the underlying stud. Hinges should be on the inside of the door. If yours are not, rehang the door or secure the exposed hinges with non-removable pins. You can do this by removing at least the two center screws of the hinge (on each side) and replacing them with non-removable hinge pins (you can get these at the hardware store) or double-headed masonry nails. Even hinges that are not exposed should be secured to the frame with 3-inch screws. Even if your door is strong and has high-quality, properly installed locks, a burglar may be able to gain entry by breaking or prying the door frame. Most door frame moldings are simply tacked to the wall, so a crowbar or a solid kick can easily separate the frame from the wall. Secure your door frames to the walls by installing several 3-inch screws along the frame and doorstop. The screws should reach the wall stud.
Install cylinder guards around the lock cylinders (the part where you insert the key). Replace flimsy strike plates. Secure exposed hinges. Fortify your frame.