Keep reading a lot. This will increase your comprehension in RC and also in CR. For SC, memorizing rules tested on the GMAT might help.
Lot of us here are non-native speakers of english,,including me. We do need to put in a little more effort for the verbal section as compared to native speakers but who said the GMAT was easy for everyone taking the test?
Check this thread
books-to-read-improve-verbal-score-and-enjoy-a-good-read-76079.htmlbb has done a great job in providing a list of books that will help improve english and comprehension skills.
By the way, is your GMAT test day nearing(within 1 month)? If so, then I'm afraid, it will hard to improve on the language so you may have to stick to GMAT strategy books and crack the verbal.