I'm going to be honest and say that I've never had this problem with content I felt comfortable with (the only reason the qualifier is there is because of a couple of tests I took a few semesters ago that I was completely unprepared for). But I do have a somewhat special routine for standardized tests.
This should be fairly obvious, but set any alcohol to the side a few days before the test. This is to ensure that you're properly rested, and getting the most out of your sleep. Now is also a good point to start the mental self-assurance (a.k.a. being a narcissist in a way that only you can know); such psych-ups will be too exciting on the actual test day, so the goal is to be as sure of yourself as possible before the test starts, so that your success is a given. Positive things only. You know you can do it. There's no doubt in your mind. You're as prepared as you can be, and you're ready to win. Because these are things in your head, even if they aren't true now, you need to make them true. As cheesy as it sounds, confidence is the key to bringing out your full ability.
Make sure you have all your stuff together and sleep like the dead the night before. If you wake up early, don't get up unless you've gotten enough sleep (you'll know). Staying in the bed while you're still half asleep will keep you rested until you're ready to get up.
If you can, avoid harsh noises or lights in the morning (and all the way up to the test, if you can). Don't watch TV; ask someone to wake you, if possible, or use an alarm that's less harsh. Use a light from a different room if it's not morning yet (though it probably will be) and keep the curtains mostly closed if it is. For me, being able to wake up gently puts me in a much better mood and so makes me feel more capable and self-confident. It also keeps me from being on full alert.
A day of winning starts with a winning breakfast! Have some eggs (I like fried or scrambled for breakfast), maybe some sausage, and maybe a piece of toast (or something else to sop up the egg yolk if the eggs are fried). The important thing is to get enough carbs, fat, and protein into your body while eating as little as possible (you wouldn't want to feel sluggish or have indigestion while taking the test, after all). If possible, get someone else to cook for you. If you can't live without caffeine, limit yourself to one cup, there's no reason to add stress where it isn't needed, but caffeine isn't your friend during long tests, in my opinion. If you don't normally drink coffee... It's probably better not to. Caffeine makes you more alert, and can improve mental performance, but you're also likely to crash before the test is over (assuming you get up even a couple of hours before the test starts); also, for me, being highly alert is bad for test performance. I normally drink whole milk, but really anything that sits easy in the stomach should be fine (because of this, I would recommend against drinking fruit juice; the acidity can mess with your stomach); of course, if you can't drink milk, don't do it, but I've been drinking milk since I was born! (lol)
Getting to the test... Again, if you can, get someone else to take you. If you don't have that luxury and aren't situated within walking distance... I don't know what to tell you other than 'try to stay relaxed'. Maintaining that morning relaxation is key (for me, at least), so do your best to keep traffic from riling you up.
If you got someone to take you, keep the interior of the car quiet, maybe cover your eyes (this helps me a lot)
Once there, get signed in and find a nice spot to sit and continue relaxing (maybe read a book if you're going to be a while, but only if you can't keep calm otherwise). Remember, you're already certain of your victorious performance, so you have no need of such things as psyching up, that'll only make you anxious at this point.
If you have to urinate, go. If you're one of my unfortunate comrades who won't be urinating for the duration of their visit to the testing center...
The actual test!
Because you're so sure of yourself and are plenty prepared for this test, you silently go to your seat with your supplies and await instructions. Your only goal is the test, nothing else matters, so continue to ignore everything else. When the test starts, begin working immediately (after all, you're a machine that takes this test); read the questions carefully, but quickly. Don't rush. As much as that sounds like opposing advice, think of it like this: you want to squeeze the best performance out of as little time as possible. Ever heard that advice about following through for baseball or martial arts? This is kind of the same: don't swing rashly at the target, don't focus too hard on one question, aim for the end of the test. If you have time remaining, go back and check your answers if you were unsure of something; don't leave any question unanswered.
I've never gotten too excited during a test, but if you do, it's probably worth a few seconds to stop and reestablish calm. However, you don't want it to come to that, and stopping every page might be too much (it really depends on the test though).
Breaking between tests (if we're talking about the JLPT or other multi-section test), you'll probably want to go to the restroom (if you can...) and get a small drink of water (at this point, unless it's a day long test, you shouldn't be putting anything that needs processing in your stomach), but remember, you're still a test taking machine, so go and return quietly. Close your eyes and reestablish calm if you got too excited during the first part of the test (I know that 'STOP' page is so exciting to see after a long test).
For the next session, rinse and repeat. The listening test on the JLPT is something that'll be new for me under actual test conditions (I can't perfectly replicate a testing center at my house), so I don't know if audio-based tests require any specific changes.
Anyway, that's all the explainable bits of how I take tests. I tend to do quite well on standardized tests. The main points are that you should be prepared, minimize stress, and keep all extraneous thoughts at bay (especially stressful ones). The general theme is 'quiet' (I don't talk more than necessary before a test).
The whole day has a weird subdued feeling to it, and I strive to replicate that feeling whenever I take a test, because that's when I'm most effective.
Edited: 2015-11-19, 6:21 pm