I improved my score in verbal from 32 in the GMAT I took on 2003 to 44 in the GMAT I took now on April 2007.
The verbal part was always easier for me probably because I like to read lot and I read in English which helps a lot.
My native language is Spanish and I came to the US to finish my MBA so this helped too.
Learning to better understand English was key.
For sentence correction:
I memorized the idioms suggested in Cracking the Gmat from the Princeton Review. For memorizing them I used the flashcards system which I learned from Leitners book (So lernt man lernen) (How to learn to learn)
I memorized the common errors suggested on the Cracking the Gmat. This helped me spot some errors which I would have otherwise missed.
I basically looked for the differences in the answer choices and discarded three of them, then I checked first for the tense, then for pronouns, then for misplaced modifiers, then for idioms. All of this mentally. After eliminating answer choices the most common mistake was tense: paralell construction.
For Critical Reasoning:
I used the strategy suggested here and on Cracking: Read the question first, then read the passage, then imagine the answer and after that look at the answer choices. Really effective.
I practiced with the PR Cracking bins.
For reading comprehension:
I tried to really understand the text. I did read all of it but always thinking about the structure of the text. I used POE on the answers, I discarded those that were out of scope. On the practice test I realized that most of my failures were on reading comprehension, therefore I took special attention to those questions, spending a littles bit more time on them and trying to really understand the text. I did mental summaries of the text like: ok, here he says that A is caused by B because...
For anyone taking the GMAT I would recommend:
Buy one basic book like Cracking the GMAT
Read GMATClub lessons
Buy
Kaplan Math workbook and use it.
Do many practice tests and analize your results.
Improving the GMAT is a long term goal and you have to do a little everyday. I took my GMAT books everywhere I went and every time I had 5 minutes I would do some exercises.
As someone here pointed: You should know what your specific weakness are when asked for them.
Maybe the most important thing that GmatClube thought me was that you could improve the Gmat with hard work!!! (and critical thinking)
Hope this advice helps.