verbal improvement

First, analyze your weak points. RC, CR or SC? Or may be you need to work on all three parts. RC takes more time and effort to improve wheraeas SC/CR can be easily improved in a couple of months.

I would suggest you to start reading newspapers/magazines ONLINE. This will improve your comprehension skills while reading on the computer screen. Solve as much official guide material as possible and make sure you apply the strategy you learn from the Manhattan Guides. Powerscore CR Bible is the ultimate book for CR. RC can be done from MGMAT guide or Powerscore LAST RC Bible (though very tough and do not reflect GMAT standrad). For SC stick to MGMAT Guide.

The Manhattan Online Mock CAT tests are very good to build stamina and their standard is of GMAT level. Make sure to give FULL Mock tests (including AWAs) as sectional tests is not what you are going to give in actual GMAT and a tired mind and brain after 3 Hrs exam is waht causes a poor performance in Verbal section even after good preparation.

Hope this helps. All the best for your preparation.
-Prabir