Only ever boo'd at the end of a game when the performance has been diar! This obviously aimed towards the team!
If you single out an individual i think you should be shouting something of encouragement! Mine are usually along the lines of; "OI, SHIT HOUSE! GET YOUR FUCKIN ACT TOGETHER YOU PRICK, MY NAN CAN DO BETTER/PUT MORE EFFORT IN.. THAN THAT!!!"
Then when they respond to my huge voice (i could shout from outside the ground and they'd hear me, should go on Britains Got Talent for having the worlds loudest voice!) i usually stand up and give it; "THAT'S FUCKIN BETTER YOU PRICK, NOW LETS KEEP IT GOING!"
I reckon if you get on an individuals back too much and start moaning at them everytime the ball goes near them or even worse you boo them, it gives them a need to avoid failure instead of a freedom to achieve and i think that is negative all round!