With weights, you can just use lower weight, and can thus freely work your muscles to exhaustion. With body weight movements, you can try negatives, but again- much more difficult.
See this diagram for example: http://www.operatorchan.org/pt/src/135399773193.jpg
The book recommended by xweb explains in more detail how to fill the holes between these exercises.
The logic works the other way, too, for increasing difficulty. Can you do 3 sets of a hundred one-armed pushups? Well, start climbing your feet up the wall!
If you can do free-standing one-hand vertical pushups without strain, I've got nothing for ya. Go ahead and start vert pressing 600 pounds. Also, try out for the Olympic weightlifting team--the gold medal should be easy!