The thing that makes shingling and roofing so dangerous is that it is an activity done high above the ground. The opportunity for injury from falling off a roof due to a misstep, carelessness, or simply not taking time to prepare the proper structures would seem obvious. But it is a fact that people hurt themselves all the time from falls from their roof-top. There is also, of course, the possibility of injury due to strain from carrying heavy bundles of shingles.
There are various scaffolding systems that you can use, ladder jacks, pump jacks, ladder hooks (used for laying a ladder vertically on a roof and securing it to the other side) and more. You can even nail 2X4s into the sheathing to give you support and to give you a foothold. Follow all safety instructions on ladders and scaffolding hardware. Even the angle a ladder is positioned against an eave can mean the difference between life and death.
For a low pitched roof, scaffolding will not be necessary, but in this case you should still be aware of sliding hammers, or other tools that could fall on the head of a passerby below. For this reason, you should secure the area at the edges of the roof.
Hammers on thumbs, cutting yourself with the utility knife, or slicing yourself on the corner of a piece of metal flashing. All this can happen to even the most experienced roofer. Be sure to dress adequately, wear gloves, and have a first aid kit handy.
Another danger in shingling is heat stroke. Keep yourself well-hydrated. The temperatures on a roof can far exceed those on the ground. Don't be cavalier about the height and do not let children or pets run around on a roof. They do not understand the danger.
Ultimately, the best safety measure is the application of common sense. If something looks dangerous, it probably is.