This is probably not news to many, especially those that have traditional alarms and pets, but I got fed up with my motion sensor triggering the hallway lights when a cat wandered past (it’s mounted at human head height) so I simply put some white sticky tape over the bottom half (of the dome). Job done.
Anyone else want to share simple solutions?
Quite a few people have used that kind of masking on sensors. Your typically find those discussed in a specific project report. Some people also put a small tube over the lens of the sensor, which can be very effective for reducing the area of detection as well.
You’ll find a lot of these on the quick browse lists in the community-created wiki. Look down near the bottom of that page for the “project report” section and then you’ll see many different lists, such as one for holidays, one for impress your friends, one for pets, one for sensors, etc. 
I think I see your problem. Take the cat off your head and the motion sensor at human head height should be ok! 
Ecolink motion sensors have a pet mode (large and small) which is extremely effective at detecting pets and ignoring them. Monoprice sensors also have a pet mode but it works a little differently. See this post for a comparative review: