The two very best ways to hurt yourself while clearing snow from your roof are 1) falling off the ladder on your way up to the roof and 2) falling off the ladder on your way down from the roof. Of course, you can always just fall off the roof directly, preferably with a dramatic attempt to grab the gutter on your way over the edge. On the other hand, the two very best ways NOT to hurt yourself when clearing your roof are 1) using a roof rake and 2) hiring a roof cleaning professional (who hopefully is much more skilled at grabbing gutters than you are).
The DIY Option: Roof Rake
A roof rake is sort of like a snow shovel with a ridiculously long handle. But instead of picking up the snow like with a shovel, you drag the snow down and off the edge of the roof, in the manner of raking or using a garden hoe. Rake designs vary. Some are little more than a pole with a metal plate at the business end; others have cool features like spikes for breaking up hardened snow or rollers for easy movement. The fanciest version I've seen, the MinnSnowta Roof Razor, is actually a push (rather than a pull) device. It cuts into the snow and sends it off the roof via a slippery "specially treated cloth" runner. This saves you the effort of lifting the head of the tool up and behind the snow, as you do with conventional roof rakes. According to the manufacturer, the MinnSnowta Roof Razor "doesn't damage your roof as is the case with a lift, chop and pull type."
Roof rakes are safe because you use them from the ground. In fact, you should never use a roof rake from a ladder, for a couple of reasons. First, unless you're a tightrope walker, you're probably not familiar with balancing long poles while standing on small surfaces (like a ladder rung). And second, when you pull the snow off the roof, it has no other place to go than straight down, right where you happen to be stuck standing on the ladder. The downside of roof rakes is that they work only on single-story houses, and some of them are downright pricey (but they're still much cheaper than fixing a caved-in roof or even a bad roof leak). The MinnSwnowta folks said that the Roof Razor can be used on two-story houses, and that they offer various tools for different types of roofs and snow conditions.
Homemade Roof Rake
If you've got cold feet about purchasing a prefab roof rake, here's a simple homemade version you might try. Bear in mind that I haven't actually built one of these, but this is the basic design I would start with: Get the longest length of 1-1/2-inch or 2-inch PVC pipe you can find (1-inch is a little too flexible). Screw a PVC floor flange to a rectangular piece of 3/8-inch plywood, and then glue the pipe into the floor flange. The weight of the tool is an important consideration, so don't make the plywood head too large. Once you've field-tested and refined your own roof rake design, you might want to paint the plywood to give it a slick coating that resists clumping of snow and ice.
Hiring a Pro
The safest and easiest way to remove excessive snow from your roof is to pay a roofing professional to do it. The service may cost well upwards of $100 per hour, but that's a lot less than emergency medical care. The availability of roofing companies that handle snow removal depends on your local climate. Even in Denver, where I live, there's no such thing as a full-time snow removal company for roofs. But in places like the Upper Midwest it's much more common.
According to Kyle Andersen at Advanced Exteriors, one of the largest roofing contractors in Denver, their pros have to set up an anchor point before getting on the roof. Sometimes this requires throwing a safety line over the roof's ridge and anchoring to it to a fixed object, like a truck, on the other side of the house. See why you don't want to do this yourself?
Andersen gets only a few calls a year for snow removal on existing roofs (that is, roofs not under construction), but his company does a lot of work involving preventing problems with snow accumulation. Namely, installing ridge vents and adding heat tape along the eaves and gutters. Ridge vents improve airflow under the roof deck, evening out the temperature on the roof and thus keeping the snow accumulation more even. Heat tape, or cables, snaked along the edges of the roof and laid inside gutters helps prevent drifts and cornices from forming as wall as preventing ice dams. In areas with significant snowfall, cornices can cause structural damage and present a general risk to people and objects below. So, advises Andersen, the safest way to deal with roof snow is to prevent a hazardous situation in the first place.