In most CR passages, there is a conclusion drawn (the main point) and some evidence (the information that supports the conclusion). Practice ID'ing both of these before moving on with your GMAT prep.

Once you're efficient at doing that, move into the assumptions. Something that is not intuitive about assumptions is that, because the author wrote the passage, the author believes the assumption, because the author likely assumes his logic is sound.

Take this very simple argument:
I play golf everyday. Therefore, I must have good concentration.
(I don't actually play golf everyday, but this is for argument's sake.)

What must I believe for my evidence (the first sentence) to lead logically to the conclusion (the second sentence)? Well, that playing golf improves concentration. If I didn't believe that, I would not have written this argument. Does that make sense?

Another simple CR point: the conclusion can be ANYWHERE in the passage. What you might see is this:

I must have very good concentration. After all, I play golf everyday.

The main point is still the same, but the evidence comes afterward.

There are many more CR tips, but I don't want to go on all day. Hopefully more people will add to this thread.