Triggering things based on physical objects moving into and out of regions

What is the best way to determine if an object/person/thing has moved into a specific space/region? Initially i was thinking of using motion sensors, but the problem here is that if the thing in question stops moving.. the motion sensor no longer sees that thing there...

I could use a camera and a computer and processing, but was hoping for something a bit more analog.

If i restrict the angle of view of a motion sensor to almost a pinhole size and calibrate the environment.. will the sensor be able to accurately tell when an object has moved into a region? What about when an object stays still in that region? (its going to be people in this case)..

any tips could be greatly appreciated.

Laser grid (the non-harming kind) and pressure pads are good methods. You can also do echolocation with 4 ultrasonic sensors. Downside to US sensors is that they would need to work at different frequencies to not have them interfere with each other.