My two cents on "proof-reading": It depends what you want to do with the proof.

• Do you want to understand why the result is true? Then don't read to proof but try to find counterexamples and notice where you fail.
• Do you try to prove something similar and want to know how some step can be made? Then jump to that step and see how it works.
• Do you want to generalize the result? Then try to divide the proof into logical steps and analyze them separately and look for room for improvement. (Of course, this is just one way to find some generalization… More interesting would be to find a totally different proof that works in a more general framework.)
• Do you just want to admire a clever proof? Then read from end to end.