You've studied hard and long, and now it's finally time to show 'em what you can do. Though I have different advice for different people when studying for the LSAT (mainly because we all think and study differently), I generally have the same advice for how to prep the day before the test.
The following 7 points worked for hundreds of students, and I'm ready to wager that it'll work for you too.
1) The day before the test NEVER take a full diagnostic. I repeat - NEVER! So many things are going on in your mind (stress and worry about the test) and body (higher blood pressure), why in the world would you add to that by sitting through a 4-hour practice test that will only worsen/increase your stress and blood pressure level?
There is yet another reason why you shouldn't take a full diagnostic test. You don't want the score you get on a practice diagnostic to psyche you out (be it a good score or a bad score). Students rarely score lower or higher than their norm the day before the real test, so don't bother trying to reassure yourself. Be comforted in your potential score and try to relax because if you ignore this advice and proceed to take an exam, and you happen to get a lower score than usual, then you will have discouraged yourself. You won't do so well on the real deal simply because you're convinced that you are "dumber." On the other hand, if you score higher than usual, you'll only have made yourself "cockier." Either way psychologically you're doing yourself a disservice. I like to preach that the LSAT is 50% logic and 50% strength of mind (psychology). You don't want to do anything to mess with your clear and focused resolve.
2) Don't do anything outside the ordinary. One of my students ran five-miles the night before her test. Typically I would commend someone for deciding to do something that'll help them go to sleep at night (i.e. light exercise), but five-miles? She ran so much, so suddenly (she typically doesn't run at all) that the next day she was sore and unable to move.
3) Don't mingle with your significant other. Some disagree with me here, but let me present my case. The day before the test you don't want any beef with anyone, especially with your significant other. Your boyfriend/girlfriend has the power to turn your positive state of mind around 180 degrees. Given that the only likely thing on your mind the day before the test is "the test," the chances of a small quibble to arise is good.
4) Prepare everything you'll need for the test and have them ready on your desk before you go to bed. You'll need at least: three #2 pencils (or one of each lead pencil and #2 pencil); eraser; admissions ticket (don't forget this! - it's on your online LSAC account. go print it out); and a timer w/extra sets of batteries (or better yet another timer - who wants to be replacing a battery in the middle of the LSAT?). You ought to also pick out the clothes you'll wear (layers) and the breakfast (your A game food!) you'll be having in advance. I know this sounds extremely high-wired, but you'll thank me when you aren't flopping around in a frenzy the morning before the test.
5) SLEEP EARLY! I say between 9:00 pm - 10:00 pm. The night may seem young, but research shows that 8 hours of sleep can do wonders for your brain.
6) Visualize and walk yourself through the process. Imagine yourself sitting there with your test booklet. Imagine the girl sitting to your left, who keeps coughing, and the guy sitting to your right, who keeps erasing and thereby shaking the desk you share. Imagine three logical reasoning sections in a row. Imagine the worst and best case scenarios. Many of you who are athletes are familiar with this process. Coaches have you do this before any big game, and there is a good reason for it. It puts you in the zone and helps you focus. You are also less likely to encounter unexpected surprises.
7) Last, but not least, keep that positive attitude. It's been proven that people who believe they can succeed will more likely succeed than those who have already admitted defeat. All of my students who are scoring in the high 160s have learned to enjoy the LSAT taking process - they keep a healthy sense of humor and understand that they can actually score high.
There are more tid-bits I've gathered throughout the years, but this should be sufficient, while by no means necessary (a little LSAT humor for those who got it).
Have fun with the test.