What to do
These precautions will help keep you safe during a yearly garage door tune-up:
If your door is electric, unplug the automatic opener so it can't accidentally be activated.
If the door is open while you are working, clamp a pair of locking pliers on the roller track below a roller to keep it from dropping.
Never remove a lift cable while a door spring is under tension; it will cut like a knife.
Never try to adjust the tension on an overhead torsion spring. Leave it to the professionals.
Sources: Dave Heck and Ken Keller