I'm trying to get yaw and pitch of the head base on my shoulder. I tried MPU6050 work well but kinna buggy at some moment and you can't really track when your head is moving or your body is moving.
I thinking of flex sensor attach on my neck to get the pitch but got no idea how to get the yaw of the head with the sensor 
Thanks in advance
need a body reference.
hard to do when every part is flexible.
your breast bone could be one place.
your shoulder moves in referece to it
your head moves in reference to it
A-B=delta
This flex sensor Flex Sensor Hookup Guide - SparkFun Learn can be used as the "primary component in creating sentient stuffed animals" 
The only thing I can think of would be similar to flex but where the resistance changes as the component stretches. I think it will be quite hard to attach sensors to the body unless you wear a helmet and attach to it and either clothing or a harness.
Can you really rotate your head 270deg?
I wore a helmet once that, when I moved my head, caused a remote robot to mimic the movement. The robot had a camera on its helmet so by moving my head I could see the environment around the robot. Unfortunately I don't know how they captured my head movement. I was only wearing a helmet, there were no sensors attached to me. I was also sitting down which removed the walking problem.
Look up remote telepresence devices/robots
Have you looked at the open source headtracker with GY85 9 axis board?
Maybe give you some ideas perhaps?