To add to bates good comments, also if you're in a time crunch, focus on the latter parts of each section.

Don't just focus on the things that are too hard for you right now. You have to get better at questions you can already do or you won't even be offered the even harder questions.

Specifically:
- You can't afford to make careless mistakes or take too much time on questions that are already in or below your range right now. You have to nail these.
- Things you learn on questions that are easier for you will help you on harder questions. For instance, you might learn to recognize the need for one particular type of mathematical manipulation on a medium-level question. Then, you might see a much harder question that requires that math manipulation in addition to others. If you can recognize the one very quickly (and do it very quickly), that saves you time to figure out the additional stuff that's thrown into this much harder question.

Having said that, this doesn't mean you have to do every problem. If you're already at, say, the 75th percentile in math, you want to study maybe the latter 2/3 of the questions in OG.