Rappelling, the act of descending steep faces by sliding down a rope, is one of the most dangerous activities a climber can do. Many climbs can only be descended by rappelling. While it looks fun (and sometimes it is), climbers die in rappelling accidents every year. Rappelling is the one thing a climber does in which she relies completely on her equipment. As such, any one error can have disastrous consequences.
Check Length of the Rappel
Before setting up a rappel, check the length and make sure that you have enough rope. If the rappel is 100 feet, a 60-meter rope will just be long enough when it is doubled through the anchor. If you are doing a multi-pitch rappel, or if you aren't sure if the rope reaches the ground, tie the two ends of the rope together with a figure-8 follow through knot, or tie double fisherman's knots in the end of each rope. Having knots can help prevent you from rappelling off the ends of your rope and falling.
Check Your Setup
You should have done this before climbing up a route, but before rappelling, check to make sure the waist belt of your harness is doubled back through the buckle. This will keep the harness from loosening when you put all your weight on it during the rappel. Before rappelling, also make sure that your locking carabiner is through the belay loop of your harness and that it is locked, and make sure the ropes are set up properly through your belay device. If you have long hair, make sure it is secured in a ponytail away from the rappel device; hair can easily get caught in the belay device, and sometimes can only be freed by cutting it loose.
A prusik loop or kleimheist knot can be tied to the rappel rope above the rappel device and clipped to your harness as a backup; this is a friction knot that will seize on the rope if you lose control.
While video of military personnel rappelling shows them jumping down the face in a series of hops, doing so puts unnecessary strain on your climbing anchors. Instead of jumping, place your feet against the rock and lower your weight onto the rope until your body is perpendicular to the rock face. Now walk down the cliff, letting the rope slide through your belay device as you do. If you are doing a free rappel where you are descending into a hole or on an overhanging face where your feet can't touch anything, try keeping both hands on the braking side of the rappel device and slowly feed the rope through.