As long as you can simulate and become comfortable with the test day experience prior to actually taking the test, you can lower the likelihood of suffering a drop.
This means incorporating the test into your routine. For example, make sure you get up at 7:00 in the week before the test, take a few full-length prep tests starting at 9:15 a couple of times, etc.
When you actually get into the testing facility, remind yourself that this is no different than a prep test - the skills are the same, the material is familiar, there won't be anything surprising, and that you're just going to stay focused and perform the same way you always do.
Also, remember that you want to go to law school, and this is just something you have to do to get there. Don't blow it all out of proportion.