Good question on how to know when to guess.
Use your 1-minute time sense here, too. For a faster Q type (SC, main idea RC), if you get the 1 minute feeling and aren't really close, guess immediately and randomly - don't even think about it.
If it's a slower Q type (CR, detail RC), then start guessing more aggressively. Eliminate anything on RC that has an extreme word solely because it has an extreme word. (This doesn't automatically mean it's wrong, but the odds are lower that it's right.) If you know the answer should be in paragraph 2 and an answer refers to something that actually appears in paragraph 1, cross it off - even if you don't understand the question at all.
For CR, if it's a find the assumption or inference Q type, cross off anything that talks about things that aren't mentioned in the argument.
Do NOT do this, though for strengthen, weaken, or explain the discrepancy. Those ones actually
should have new info in the correct answer.
In other words, know how they set traps for you and know what kinds of characteristics the right answers should have depending on the question type and sub-type. Use that to guess aggressively, then move on.
Okay, next. You'll be a little stressed out on exam day - we all are. :) Just don't get upset with yourself for being nervous - that makes it worse. It's entirely normal to feel nervous. Take deep breaths, roll your neck and shoulders around, and tell yourself "this is only one question."
Yes, do start familiarizing yourself with IR. A lot of people this past year have made the mistake of ignoring it and then spending WAY too much time / mental energy / focus on it during the real test - which then kills quant and verbal. Know the strategies for each IR Q type, know which ones you find easier vs. harder, know how to guess. You can guess completely randomly and immediately on 2 to 3 of those (since you don't need a great IR score). Just save all that time on whichever ones look really impossible when they pop up and use that extra time to reduce the stress on the rest of the section.
Just got to your bit at the end about stressing yourself out. If you need to, take 15 seconds, close your eyes, flex and stretch all of your muscles everywhere. Then look at the screen again and try to do that problem. If you start fixating on the idea that "I can't focus" don't tell yourself not to think that. It's impossible to tell yourself NOT to think about something. :)
Instead, tell yourself, "I can think about that all I want in just one minute - as soon as I pick an answer for this one, I can think about whatever else I want." Distract yourself from the distracting thoughts!
Also, on the break between quant and verbal, drink some kind of liquid that is high in glucose and has ZERO high-fructose corn syrup. Coconut water, fresh fruit juice, whatever sounds good to you. This will give you a short term mental boost that can help to combat some of the fatigue at that point on the test. (The reason I'm not suggesting that you do this before or on the first break: glucose is always combined with at least some fructose, so there's a "crash" factor eventually. I don't want you repeatedly peaking and crashing throughout the test.)
I love Ron's opener drill - it's basically just looking at a bunch of questions and asking yourself, "What's this about and how would I do it?" without actually doing it. You can use this with new questions or ones you've already done, as long as you don't outright remember them already.
If you haven't done them before, then go ahead and do them to see whether you were right about your planned approach. You can also try doing the "mini-quant" sections at
www.gmatfocus.com - there's a fee but these are real past test questions and they do give you timing and other data. Note that they don't give you a time limit, so set a timer for yourself (48 minutes because there are 24 questions).
Have you done GMATPrep yet? There are 2 and you can probably repeat them once without seeing a crazy number of repeats. That should be plenty - you shouldn't be taking more than one exam a week anyway. I haven't tried 800Score in a while, but those and Kaplan seem to be the next best (after ours :)
Good luck with everything!