My property prof did a nice job of explaining the RAP last year. From my outline:
1) State the title
2) State the condition precedent (what will make interest vest?)
3) Find a tentative measuring life (person whose death resolves the contingency immediately or at least within 21 years)
-if that person is a class, it must be closed
4) Is tentative measuring life in being at the time of grant?
-if not, interest is void
I'm a fan of the create, kill, and count approach. Easiest to remember!
That's the way our professor taught us. Basically, draw the family tree as best as you can glean from the problem, start killing off all the lives in being at the time of the creation of the grant, and ask "what might happen within 21 years of the last person's death?" I found it was pretty effective.