In most houses there are numerous points of entry an intruder might use. Just having an expensive mortise lock on your front door really isn't enough, and you need to ensure that all possible entrances to your home are secure.
A standard casement lock on a door will not stop an intruder for more than a few moments. A burglar will ring the doorbell and when there is no answer, a swift kick in the right place could have them inside in seconds. Fit a good quality deadlocking bolt (a mortise lock for example) and consider fitting rack bolts top and bottom, especially if you are away from home often. If you have a UPVC front door or are considering having one fitted, check that it has multiple locking points (most do).
Also consider fitting a strong security chain as burglars have been known to wait for the door to be answered before forcing their way in. Thankfully, this is not common, but a chain will certainly make lone occupants feel safer when answering the door at night.
Increasingly, thieves are using a method called fishing. This means that they will use a pole with a hook on the end to "catch" keys left on hallway tables. This could be you car keys, house keys or both. Never leave keys anywhere near the front door, unless they are secure in a drawer, etc.
The back door is even more appealing to intruders as they can often work unseen, meaning that they can take their time. Your back door should be at least as secure as your front door, with good quality deadlocking bolts in place. Try to trim back any trees or bushes directly around the back or side doors, reducing the burglars ability to hide.
Some older sliding french windows can be lifted out of their tracks, even when locked. Fit additional locks to the frame to prevent this. With wooden french doors, fit rack bolts top and bottom, close to where the two doors meet. This is the weak point of any double door.
Never leave windows open, no matter if you are just popping to the shops for 5 minutes. Fit additional locks to all windows, especially to the rear and side of the house. Rack bolts can be fitted to any wooden windows quite easily and you can buy locking systems for UPVC and Metal-framed windows. Make sure that any locks have removable keys, otherwise the burglar can just break the glass and unlock the window.
Most upperfloor windows will be safe (unless you leave ladders laying around) but any that can be reached by climbing a drainpipe or from a flat roof should be secured as you would with a downstairs window. If you think a downpipe could be used as a way to climb to a window, you can paint it with security paint. This clever substance remains slippery, making climbing very hard.
Not many modern houses feature louvred windows, but some older houses might still have them. The individual slats of glass can be easily removed by bending back the metal frame that holds them. One way to prevent this is to use a strong epoxy resin to fix the slats in place in their frames. The only other way to make louvred windows secure is to fit a security grille.
Not really a problem unless the roof can be accessed by means of the downpipe or lower flat roof. Fitting a simple rack bolt will make skylights secure enough to deter a burglar.
Not a problem in detached houses, but it has been known for burglars to break in to one house in a terrace, get into the roof space and break through the dividing wall to the next house. Indeed, some older terraces (and even semi-detatched houses) have a shared roof space. Fit a standard bolt to the hatch to ensure this never happens to you.
Outbuildings can be both a direct target for a thief and a useful place to find things to help them get into the main house. Fit a good quality lock to sheds, preferably a hasp and staple which when closed covers the screws. For garages, fit a heavy duty bolt which can be padlocked.