Get in the practic of tying a knot at the end of your rope even if you know there is or you can see several feet of it on the ground.  Because that day just might come that..."WELL YOU KNOW."
Just be glad that you are in this age of devices the pictures below will tell you why, but they are good to know just in case you suddenly heard that dreadful ting, ting sound.  If you look around you will see at least a dozen different types of combination rapell and belay devices that can be used for rapelling or just a rapelling device. Most every company has their own design and designs. 
The first thing I want to say is you better go slow very slow, because you are using your body for friction. 
This one is good to know if you hear the PLING, PLING, PLING!

Picture this'; You step over the rope so it is going between your legs.  Then you bring the rope around your right hip across your chest and over left shoulder.  Then across your back finally holding the rope in your right hand. That will be your breaking hand and your left hand is you feeling hand.  Then as you rappel down the friction heat builds up,  well I will let your imagination take it from here.

This one is called the Dulfersitz Rappel
I have seen allot of rappel devices changes from those times.  From the Carabineer Break, Carabineer Break Bar, Carabineer Wrap and the Stitch Plate, etc.  From the latter came the multitude of different designed Rappel devices.