Aside from all the drills posted here.. i think the most important bits to the kip are
1- Strength, you need in my opinion at least 3-4 good leg leg lifts before you should be working on spotting kips
2- Work glides and drop kips to DEATH!!! get both reallllly good (even if they can't do the drop kip but can understand it and when you are spotting them you can feel they understand there is a timing issue, and that they can keep the feet to bar by self)
3- V-hangs for time with weights + L-hang, leg lift, shoot up to hollow hold (like the bottom of the backhip circle)
These are the key things...only after they can do that, i will start spotting kips all together. I personally will use a few drills just to teach transfer of momentum, and shoulder flexion (closing of the shoulder, and i know that some of you will call that extension)... Walla... You need to be strong to get the timining, but you don't need a great deal of strength once you have the timining.. that is the issue.