[unsung]

I would strongly second the GP's recommendation. The book mentioned is aimed at math teachers up to an undergraduate level, but generalizes quite naturally.

The idea is that one may induce a path to solution in a student by stripping a problem down to it's knowns, unknowns, and constraints so that it may be comprehensible, then connecting these elements back together by asking pointed questions that could have conceivably occurred to the student. He gives plenty of practical examples of how one may dial the complexity of these questions up and down, how to relate by analogy, and plenty of other techniques for approaching problem solving generally all in a flimsy paperback that can be finished in two evenings in a weekend.

The most impressive part, to me, is that nearly every example is both illustrative enough to be a word-for-word guidebook for math tutors, and accessible enough for even a middle school student studying Euclid to understand in it's entirety.