So, i got a 4 on my diag verbal... then slowly increased to an average of 9... still hopefully rising. Mapping INITIALLY helped me increase to like 7's or 8s... but then I realized its just a means to an end. You should eventually not need to physically write a map, but still do it in your head as you end each paragraph. And if you finish an essay without knowing what it was about. Reading all the question stems right after (a kaplan strategy) actually will help point out the main idea sometimes.
But yea... in all honestly, i started doing better when I just faked that I LOVED the subject, even if i didnt. THen I read the whole thing, WITHOUT re-reading a sentence I felt I didn't understand (keep reading, you did catch it, you don't realize it)
A third strategy thats working (but it sounds bad when you think about it) is that when I do the questions, I try to pick an answer without even going back to looking at the passage. Looking back will cause you to pick the "faulty use of detail" answers because you'll see an answer that looks like the sentence in the passage even if its not right....