Well a coach can certainly help provided he/she is well versed with the GMAT format, typical traps so on and so forth. The problem in India at the moment is that most of the coaching institutes are not familiar with the GMAT format. They end up fleecing money from GMAT aspirants and re-use the material from the OG.
Coming to specifics, I totally agree with Mayank that a minimum of 12 weeks need to be kept aside to prepare for the exam. Must have books are OG, Princeton Review Cracking the GMAT, Kaplan 800. I somehow found the Manhattan Guides a big letdown considering the costs associated with them.
Its extremely critical for an aspirant to have an evaluation of strong and weak areas. When we talk about strong and weak areas, don't just limit it to Quant or Verbal. Break it down further. What kind of questions in Quant? Are you having issues with Problem Solving or Data Sufficiency?. If it is Problem Solving, which concepts are you facing the problem with? Algebra, Mensuration, Geometry, Sequences, Probability?
In the Verbal, Sentence Correction, are you having challenges with Idioms, Pronoun Errors, Tenses? Once you start noting these down, you need to pick examples. At the end of each day, review full statements of problem types you have an issue with.
Good Sources of Problems for Each Type
Quantitative:
Problem Solving: Kaplan 800, Kaplan Comprehensive, Ace The GMAT
If you can solve these problems accurately within 2 minutes each on an average, be assured that you will score well on GMAT
Data Sufficiency: OG Complete, OG Quantitative, Kaplan 800, Ace The GMAT
Verbal:
Reading Comprehension: OG Complete, OG Verbal, Princeton Review Cracking the GMAT, Princeton Review Verbal Workout.
Most of the passages in these problems are from the paper-pencil GMAT tests. The passages are longer and you have quite a few questions per passage. If you can do these well, no doubt that you will do well on the GMAT. In the current GMAT format, you get around 4 short passages with 3 to 4 questions.
Sentence Correction: Same as Above
Critical Reasoning: Same as Above. Also go through the arguments in LSAT preparation material if you have the time. The OG questions are good for practice but both OG and Kaplan are a bit of a let down in CR for higher score levels. Princeton really does the trick as the material is more diverse in breadth as well as depth.
Sentence Correction: Same as above. If you can do OG, Kaplan 800 and Princeton materials, you are in for a higher score.
Note about Kaplan Tests
The Kaplan Tests are extremely good for practice. However, don't read too much into the scores as they are designed to be low scoring. The most accurate indicator of your test score is your performance on Princeton Tests.
I don't give weightage to the Verbal Section of the Prep Tests as most questions are repeated from the OGs. So you actually 'know' the answer to most questsions from preparation without having to 'solve' the problems.
: here!