People who can't do many pull-ups usually turn to some type of assisted pull-up machine. But most of these machines don't go high enough to allow the chin to clear the bar, and none of them work on the capacity to move your body in space since the body is fixed (no tridimensional movement). A better method? Stage reps.

How to Do Stage Reps

1. Bring yourself up to the chin-up bar by stepping on a bench. Lift your feet off the bench and do partial reps at the top – only go down to a position you can get yourself back up from – for 3-5 reps.
2. Now lower yourself down to the bottom position under control and do partial reps from the bottom – from dead-hang to as high as you can – lowering yourself under control. Do this for maximum reps.

Over time, the length of each phase will increase and soon you'll be doing full-range movements.

Bonus Method: Horizontal Rows

While these don't involve the same movement pattern as the pull-up, they still make you practice lifting your own body. There will be some skill transfer to the pull-up. This isn't my primary strategy, but it can be added since it's not too stressful on the body.