Don't leave anything out in the open that can give away what else might be in your car. I made that mistake. Some of the wires leading to the trunk from my HU had fallen down in the passenger footwell. Someone saw them, realized I had equipment in the trunk, and smashed the window in. Also, disable the trunk open by the driver's seat. Stick to using a key or fob. It isn't 100% secure, but it does help.
