Negative muscle-ups!
In case you're not familiar with the concept, basically, a negative of an exercise is starting from the finished position and slowly going back to the starting position.
Find a ledge that you can get your arms on, elbows straight, legs hanging, with relative ease. Start from the up position, and as slowly as you can (which wont be very slow at first...) lower yourself down until you are hanging from the wall.
It can also be done on rails like
this guy.