Is English your first language? If not I would consider an English tutor.
If it's not, then you can probably improve on your own with steady prep, or with a GMAT tutor just for verbal. I agree on PR (Princeton Review
) for verbal...the strategies are dead-on and their practice test questions are accurate, very much like the real thing.
I have not used Manhattan's guides (I am better at verbal than quant) but have read good things especially about the Sentence Correction guide.
Ooh -- another book that is good for verbal is called Grammar Smart. If covers the most common grammar issues in the English language. It's a keeper for essays too.