Attach a roof anchor. Roof anchors can be installed easily and guarantee safety. They are often required by OSHA. Use deck screws (not gold-colored general-purpose screws) and be sure the fasteners penetrate a framing member, not just the sheathing.
Anchors like these attach by driving large screws into the ridge board or the rafters near the ridge. Others require that you cut a hole in the sheathing and wrap the anchor around an underlying rafter.
Clip on the rope. Use heavy-duty braided polyester rope and approved hardware for clipping to the roof anchor. Make sure the rope cannot come loose. Ropes like this one have a built-in shock absorber to cushion a fall.
If you are working below others who are roofing or tearing off, wear a hard hat. Use eye protection when tearing off or driving nails. Roofers often wear gym shoes to help them keep from falling; a pair of rock-climbing boots is the ideal footwear. Long clothing will protect you from scrapes and the sun, but you may prefer short sleeves and shorts if it gets really hot. Invest in a high-quality pair of knee pads that won't chafe the back of your legs.
Strap on a body harness and clip on the rope. If you use a self-retracting lifeline, the rope will stay taut so you won't have excess rope to trip over as you work.