In reply to FockeWulf:
Like Whitlew said, rather than tying a single rope to and anchor and abseiling off that, you feed it over/through/behind something at the middle of the rope, toss both ends down the face you're abseiling, abseil down using both sides then pull on one end of the rope to pull it down. This is one of the reasons ropes often have middle markers.
Of course this means that, unless you're using some slightly advanced techniques, you'll need a rope that is double the length of the longest abseil you'll make.