My motion dectors came with instructions
and a "stick on" masking device that allows
you to create a "pet zone". Basically it
masks-out the dector from seeing motion
near the floor. You can also set the
switch that requires "double motion" before
activating (smaller things/pets are less
likely to activate in that mode). It
reduces the chances of a false alarm (but
also reduces effectiveness).
Here's another suggestion, set the alarm
using the mode that ignores motion
detectors. Use that mode for those times
that you need to leave the pets roaming
around.