Assisted Pull Ups and Ring Exercises
An excellent method to perform assisted pull ups is with the use of a pulley and weight. Picture above, the method uses a built in lat pull machine. Two pulleys could easily be attached to the ceiling, by the pull up bar, and would work the same. The main benefit of this method over others is it is possible use exact weights, with infinite adjustability. A belt is placed around the waist or shoulders, both work, but give a little different feel. A short bar allows the weights to be pulled into place and hooked into the belt. This method replaces a $2000 Graviton!
Other methods include the use of bands (see McMaster-Carr for cheap bands. Search for pallet banding. Currently they are on page 1396). And self assistance using a chair, or a helper.
All methods can also be used with gymnastic rings, to perform a variety of exercises.