I noticed a big improvement in RC when I started taking notes while reading the passage. The notes weren't long, or detailed, or even complete sentences. Since the passages are so dense, I would write a couple of words or phrases relating to what I was reading. Basically, sum up the paragraph (or where a natural paragraph would be if it were normal reading). I hardly ever needed to refer back to the notes to answer the questions, but the act of writing the ideas enabled me to organize the passage better in my head.
The downside to this (especially if English isn't your primary language) is that it adds a bit of time to answer the questions. It wasn't a problem for me, I didn't run out of time, but English is my primary language.
I improved SC through brute force--just mindless repetitive studying, to the point where I basically memorized all the answers OG11
and whatever else books I used. The answers explain "why" something is right or wrong, so if you read them enough, (and have a good memory) then you can get a handle on pretty much 80% of everything they may ask.
As for CR--Assuming that your English is well enough to fully comprehend the subtleties of the passage, one thing I suggest is to read biased material (newspaper editorial, opinion columnist, political writing, etc...) and look for flaws in the argument, or think of what might be needed to back the argument up. If someone has a bias, then they're probably going to leave information counter to their beliefs out of the writing, or "spin" the facts presented in their favor. Look for what isn't there.
Everyone is different, but this worked for me.