Having someone in the house turning lights on and off throughout the day (as well as exterior lights) gives the illusion that you're home. Setting a timer works, too, but a live person using different lights throughout the house at different times is much more natural, further deterring anyone from messing with your place. The green bonus is that your sitter will probably use less energy with lighting than timed lights would, while giving a more convincing appearance that the place is occupied — and off-limits to crime.