If you keep a spare key outside to prevent yourself from being locked-out, think carefully about where you hide it. Doormats and door frames are predictable places that burglars will always check. You should hide it in your dog's collar.
If you are going on a vacation, ask a neighbor to pick up your medical id badges mail and check your home for flyers. Canny burglars will often produce and distribute their own fliers so that they can see whether or not anyone takes them down. Fliers that are not removed let the burglar know that the home is vacant.
A great protection against burglars is owning a dog.