( *EDIT: oh I thought you were talking about the Spivak book, munkres is ANALYSIS on manifolds, get it straight :P ) I know how you feel, when I first saw the "inverse function theorem" proof in Calculus on Manifolds , I was just like holy schmoly I'm not reading that! But anyway, a good way to read long proofs is to do the following: before you fully digest the proof, you must fully digest the hypotheses and the theorem statement itself!
With that in mind, you can section off parts of the proof according to what parts of the theorem statement must be satisfied before pushing forward. ( for example, show that x is linear and invertible; look for the part that proves linearity, then look for the part that proves invertibility. ) This is especially easier if the theorem has multiple parts (i.e., if blahblahblahblahblah, then f is differentiable, continuous and ____ ). Also, you must look for the parts of the proof that uses each part of the hypothesis. Don't let any of the hypotheses go to waste! And if you end up realizing that certain parts of the hypothesis were not required at all, then that's cool too -- you'll probably understand the requirements of the theorem better, and you'll get a feel for what is redundant and what is not.
Skipping proofs is useful sometimes; whether or not you should read the proof depends on how much "structure" or concept of something the proof itself can reveal. If you care about x and y being so and so, there is a more conceptual quality behind those properties, and you could care less if you had to use the triangle inequality to prove such a thing. However, you *should* always be able to prove everything you know (this doesn't mean you aren't able to momentarily skip proofs )