Critical Reading is the hardest section to improve on, but improvement is possible nevertheless.
What I basically did when I was in your position, was continuing to take a lot of practice tests, and reading challenging books like Gravity's Rainbow, Infinite Jest, Ulysses, etc. Although looking back, I may have fared better if I chose to read stuff I was actually interested in, like my pop-math book.
The SAT is different from how English classes gauge your reading ability in that there is no subjectivity. The questions aren't "open to different interpretations," that is--there is only one right answer, and while there may appear to be other right answers, they're merely the ETS trying to trick you. It is a "reasoning test" in that you have to use your reasoning ability to decipher through the trickery, and find what is objectively the only "interpretation."
I hope what I typed helps you in your endeavor. I'd wish you "good luck," but luck is for chumps who can't get by without good old fashioned hard work.