Now that I am done with the test, I can give some advice.
1. Analyze what went wrong - Is stamina your problem? Do you have your fundamentals clear? GMAT tests a few concepts over and over. If you don't have clear fundamentals - you will feel like you hit a WALL when faced with a difficult question.
2. Understand your weaknesses and strengths - Yes it is important to know where you stand. Only when you know how good or bad your are in certain areas - you can improve.
3. Work on your weaknesses RELENTLESSLY - Practice a lot of questions. Analyze your mistakes and learn from them. Keep an
error log and go over the log several times.
For learning the fundamentals lots of people have recommended
Manhattan GMAT guides. I used the word translations and Sentence Correction guides and I don't regret buying them at all.