What do I do if I forget or drop my rappel device?
To get down you have two good options and a few mediocre ones:
If you have a GriGri or other similar assisted braking device, do a single strand rappel using the biner-block (Reepschnur) method.
Rappel with a Munter hitch using both strands of the rope.
Less good options:
Have your climbing partner lower you down the pitch. If there are multiple rappels involved or communication is difficult, this is not a good choice. Your partner may be unable to place you at the next rap station and there's a chance of getting stranded.
Use a carabiner brake rappel. This option is finicky to setup and — to really work as you'd like — requires an arsenal of oval carabiners that you're probably not carrying.
Down jumar the pitch using prussiks. This takes forever.
Use a dulfersitz rappel. This is your last ditch option if you don't have any carabiners.
Practice all these methods before you ever have to use them.