Please try reading, atleast once daily, below links
Business News & Financial News - The Wall Street Journal - WSJ.com
The New York Times - Breaking News, World News & Multimedia
There are some nice articles and they will expose you to a variety of topics such as business, economics, science and many more.
Also start studying the manhattan guide sentence correction guide and try applying it everywhere you write and read officially.
Many times this has helped me figure out why a sentence is redlined in outlook or Ms word.
Make sure you have a reading speed above 300 wpm. You will be able to find the speed reading technique book somewhere in this forum. Practice it atleast once daily.
At the end of one month you should have a good reading speed and good grammar knowledge. This is not only neccessary to ace the test but also will help in life during MBA and beyond.
For vocab, there is software that keeps popping up a word and its meaning every n seconds on the bottom of your computer. Install and use it, you will be atleast exposed to a variety of words.
Hope this helps