Here's how you can modify the T-Rex neck (of the T-Rex in set 8485) to make it shake it ('just say no'). In red, my modifications; micromotor required!
Now you do not have enough switches on your control panel to control the neck. I made a switcher between two motors, and connected it to the A/B control ports; so you can control either the sideways motion of the neck and tail, OR the shaking motion.