Eat a healthy, large meal before arriving at the testing center. You don't want to be hungry while taking the MCAT.
Plan on staying in the testing room during the entire MCAT examination. While you can leave the testing room during your breaks and eat a snack, it is a hassle to do so. You have to check out, and check back in with biometric data (fingerprints). There could be a line of people trying to check back in. Instead of stressing about whether or not your break is up and your MCAT timer is counting down on a section, avoid the situation completely.
Arrive at least one hour early. Make sure to bring proper identification. Double check your AAMC MCAT emails about what exactly you must bring with you. Getting there at least an hour early will allow you to sit, get a handle on your nerves, go to the bathroom, fix any problems (if they arise), etc. The last thing you want is to feel rushed walking into the MCAT exam room.
Take a deep breath before starting your MCAT exam. Smile and tell yourself that you are ready. It's important to have this positive reinforcement!
If a question seems overly challenging, look away from the computer, and take a deep breath. Move on to the next question if you need to. There is a button you can press that will let you see what questions you haven't answered yet.
Take your full breaks. Breaks are only 10 minutes long. You need to let your mind rest. If you don't take the full 10 minutes, you may regret it. That time is lost and is not added on to your other breaks.
Never finish a section early and then immediately move on to the next section. Always run out your time while reviewing your answers. While you take the test, you can flag certain questions you are unsure about. Make sure to review them if you finish your section early.
Move on and forget about a section if you don't think you did well. There's nothing you can do now, except for performing well on the remaining sections. Don't let it get you down! Letting a section get to you is more damaging than performing somewhat less than stellar on only one section!