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As others have said...
1) Reinforce what you can to make it a little harder to get in. Door jambs, pointy shrubs under windows, etc.
2) Motion sensing lights in strategic locations.
3) Video cameras are a good investment in strategic locations. They have entire kits with cameras and recorders together.
4) A dog of some kind (preferably larger, although often the bark is enough as it alerts everyone that something is possibly amiss).
5) Be friendly with your neighbors so they tend to look out for you.
6) Alarm system.

You can't make it impossible for a determined individual, but you can most often make them want to pick an easier target of opportunity. I would not put up any signs, other than a generic no tresspassing, and maybe an alarm company sign. Certainly nothing that implies you already have forethought of shooting someone. Even if intended only as a scare, a DA / ADA would likely frown on that if you ever were forced to defend yourself or your family at home.