Hi all,
I've been stalking the forum for a while before taking my GMAT and found some very good study material on here so I though I would give back some experience I got from my study periods and test taking.
Overview, I take the GMAT only once in February 2015. I study for it on/off for almost half a year (with two 3-month gaps in between so it's almost one year). My initial score is 680 (better math than verbal). Some background: I'm Vietnamese, study in an international university in Vietnam.
Tip and trickUnfortunately I take a course in my hometown (HCMC) which used its own material which I notice is a combination of a lot of sources stated on here (Mostly Kaplan, Princeton, Manhattan and Official Guide). However, there is a few lesson that have been drilled into me which I think you can benefit from:
AWA: don't study for it too much (spend 2 days reviewing the list topics + chineseburned awa got a 5 in the end) focus more on the Verbal and Quan, that's the key.
IR: also not a focus. I did familiarized myself with the type of question. (got an 8) The biggest lesson for me is APPROXIMATION IS ENOUGH. Don't bother with the complicated maths, round number to the nearest reasonable number (ex: 4985 ~ 5000) before calculate and pay close attention to the question. That's where the trap lie.
(My teacher actually tell me to take it very easy on these two sectors as there don't count into the final score and wear you down significantly, especially the IR).
Verbal: Timing: 1-10 (25 mins), 11-20 (15 mins), 20-30 (15 mins), 30-41 (20 mins)
1.
Sentence: : Analyses the original sentence very carefully, break it down to Subject-Verb pair. I found that half of the time S-V agreement (plural-singular, tense) is reason to eliminate half of the answer, then pay attention to adjective clause/phrase (again tense and subject-verb agreement), object (especially does it match the subject). Usually you will be left with two - three choices after consider all the above. Pick out the differences between these choices and decided which one is correct. By strictly following these step, I save a lot of time (average time for sentence question is 1 min or a little bit less). It helps me make sense of the question since English is not my mother tongue and sentence is my weakest point from the beginning.
2.
Critical Reasoning :
Read the QUESTION first, no exception. Pay attention to two points: what type of question (Weaken, Strengthen, Infer, Bold face...) this will help you frame the direction of your thought, what other pieces of information do you need? (usually it will be a conclusion, a plan, a hypothesis... of somebody). Skim through the paragraph until you see this information (and I mean skim, don't read, don't pay attention to anything except to look for that exact word (usually GMAT use the same word in the question as in the paragraph) and usually it's at the end of the paragraph). Read that sentence carefully. If it mentions another piece of information (ex: this substance, this plan...) then go back up and look. Only after those steps did you start reading the answer. By this time you will have a good idea of the question without reading it from end to end.
Eliminate the answer base on scope first. Usually, 2-3 of the answer can be eliminated this way. For example, if the question is about country X and answer A and B mention country Y or "other countries", they are out of scope and is eliminated. If the question is about cost and the answer mention method (without linking it to cost), it is eliminated. This process is hard to explain and is scary to apply (in fact the whole methods of doing Critical Reasoning this way is a bit scary because you are leaving out pieces of information on the question which you don't read and you are judging answer based on a few word in it) but it have served me very well. It saves time and require little effort. If you want and example, post a question below and I'll solve it this way for you.
3.
Reading : This is the part where I don't really have special trick. Usually it require the same strategy that I used in Critical Reasoning. I did make a note to skim through the whole paragraph first (there's usually a general question that require you to give the overall theme/topic anyway). I pay particular attention to the first few sentences (it usually point out the topic) and any transition words (ex: however, furthermore, first of all, secondly...) which will allow me to understand the structure of the paragraph (same as critical reasoning, there are part that I will not be reading since it does not come up in any question)
Quantitative: I'm sorry but I can't help you much in this area, my math score goes up quickly after a short review of the basic maths concept and solution. I do have an advice on timing though 1-10 (25 mins), 11-20 (15 mins), 20-30 (15 mins), 30-37 (20 mins). During the one minute they read the instruction, I divided my scrap book as below: Start from the beginning, first two page divided into 10 section (5 per page, usually two at the top, two at the bottom, a narrow section in the middle) at the end of page 2 I write the number of minute I should have left (which is 50 mins). Repeat it twice for the other two sectors and for the last sector, each page are divided into 4 with the last section on the last page cross out. This allow me to keep track of which formula is for the current question (thus allows easy check) and keep track of time. If you get to the end of any section more than 2 mins early then slow down, more than 2 mins late then speed up. If you are 5 mins late to any section, just randomly tick the next question and speed up to catch up.
Making habitI know I will most likely take test very early in the morning (in HCMC) the test start mostly around 8-9h in the morning, depend on the time you book. So I mostly do full test on Sat morning, starting at similar time. I think it creates a good habit. Plus, my other time slot is after work which gives horrible result (usually a 50 point different since I'm burn out right at the start). I also notice that being hungry or recently eat a big meal reduce score too

. I'm completely serious, the difference is big enough and constant enough to be noticed.
I build a healthy habit in the week before test day: short exercise in the morning (30 mins), full breakfast at least 30 mins before the time I suppose to take the test (8 a.m.). Because I work full time, taking a long break to prepare is not possible (I got 2 vacation days left at the time). So I take those two days off, the first days are for normal practice and one full test (Princeton: 730. This is also the first time I take the test with the AWA). The second day (one day before the test), I follow my routine then start the test at exactly the same time as the official test (GMAT Prep Test 2: AWA, IR, CR and Quan. Get 770, I don't remember exact score but Q is 52 or 53 and V 46). The rest of the day, I relax, watch TV (Director cut of The Two Tower and The Return of the King, my brain is mud by the time it's end

).
Test day:I follow the exact habit I make the day before. Between section, I drink Coca Cola (I'm not advertising for them but since high school I drink them if I need energy quick (I hate coffee)) and do some simple stretch. Most importantly, don't bother thinking about questions or sections that you have done. It serves no purpose and slow you down. If you get an easy answer after a difficult one. Don't freak out and think you get the previous one wrong. It is most likely an unmarked question (I don't remember what they was call but it's the one put in to test if the question can be used in future test). By the end of the Verbal, I can feel myself slow down and the question start to make not much sense. But I have over 10mins left and only three question (none of which are reading) so I relax and I go slow and careful with them. I think I got them all.
I believe the GMAT isn't supposed to be a test of your reasoning still alone but rather how you manage your time and effort to product the best possible result. Except for extraordinary cases, most of us probably left the test room exhausted and with a headache. So it's a matter of maximize your resource (hence the habit) and utilize that resource well (the crazy strategy).
Practice test:You probably have found lots of good advice on what practice test to use elsewhere on the forum so I won't repeat it here. I do suggest you leave Test 2 of GMAT Prep for right before you take the real test. It is probably the best indication of what you will face on test day.
Hope this help.