Tip1: Data Sufficiency..
Treat options (B) and (D) as red flags..
Every time you’re marking an answer as (B) or (D), double check it. No need to spend  any extra time, just make a mental check.
Many times, we mentally drag down the data given in condition 1, while evaluating condition 2.. And when we find (2) to be sufficient, we confidently proceed and mark the answer as (B), whereas it shud’ve been (C) 
From the discussions on GMAT groups and my own experience with Data-sufficency Qs, I can tell u that this is a very,very common problem.. I too did this all the time, while I was doing OG. But once I made this rule, I never ever committed this mistake again.

Tip2:: GMAT Verbal:
In quants, if you spend 5 minutes on a problem, chances are that you’d get it.. Not that I actually recommend it, but what I want to emphasize is that you can get to a correct answer in PS and DS if you spend more time. This, however, is not the case with Verbal. After spending some 30-40 seconds on a question, you often end up with two options and sometimes it’s impossible to decide which one to choose.
For example: You’ve read the RC passage in full and have almost complete grasp of the content. The question that comes up first, asks you the author’s tone. You’re sure it wasn’t happy or appreciative. It can’t exactly be termed as sad. Now you’re down with the final two options: indifferent and angry. And you feel that the author looked indifferent at times, but a bit angry at others.. Normally at this juncture, you spend a good amount of time, juggling between the options, scrolling up and down the passage or just staring in blank.. Suddenly after 5 minutes have passed, u realise that u have many, more questions to follow. You quicly pick one of the options and proceed to answer the next question.. Just think, couldn’t you have done exactly the same thing 240 seconds earlier??
Same applies to CR and SC questions too. If you are not sure whether the correct idiom is “mistook as” or “mistook for” and u have already tried fitting this idiom in a simple sentence of ur own (highly recommended!!), I don’t think that you’ll get to the correct answer confidently even if u spend 10 mins on the question.. 

The crux is that once you’ve exhausted your knowledge of idioms, grammar and common sense, you’re not going to get the correct answer by spending more time… So as and when you realise that you’re not able to choose between two equally appealing options, just pick one of them (which appeals slightly more) and move on.

Hope you find these useful.. In case of confusion/need for clarificatin, feel free to leave a comment or drop a mail qo my gmail ID: quizophobic.