Thats a lot of time, good for you!
1) Try reading very good newspapers and magazines- economist
2) If you are worried about your grammar/idioms, try gettting a good grammar book and reading it thru chapters - make notes - refer with internet after each grammar lesson.
3) There are some techniques for 'speed reading' - you can search the web and improve you RC skills.
PS: It has been a while I wrote GMAT, so pls take this advice with a pinch of salt...