Get a motion sensor that screams if someone tries to move it.
In all seriousness, I've just generally tried to keep trips short and keep my bike in areas where I can see it through the window. If I'm going to one place and staying there for several hours (like a movie), I'll carpool with friends or take the bus. Taking it to work or school, I'll discuss the safest and most out-of-the-way place to keep it with maintenance and custodial staff. Knowing that the security guard walks by the bike racks is pretty fair peace of mind. But if I can keep my bike in the break room or a storage closet, so much the better.
As in the way of Miyagi-do, the best block is "No be there."
