I haven't written the LSAT but here are some things I make sure I do when I'm heading into a stressful exam, meeting, etc.

Keep your schedule! start going to bed and waking up when you will have to wake up for the LSAT so that on test day you feel rested. This will also condition your body to at least try to start falling asleep and often helps mitigate against that "OMG I CANT FALL ASLEEP AND I NEED REST" moment some get before an exam.

Don't do anything strange with your diet. Stick to what you regularly eat to avoid any digestive issues during the exam. I also believe that creating a similar atmosphere internally and externally to what you will face in the exam helps with memory and stress levels.

Look at the exam for what it is. Know that you have prepared for it and that although the questions might be slightly different, you have seen it before in your practice exams. You should have a fairly clear idea of how you are going to do before writing the exam, if you're happy with how you have performed in practice exams, let that comfort you -- YOU ARE READY!

I also have a tendency to feel nervous right up until the moment I start writing an exam. At the point of writing I become focused on what I'm doing. Reminding myself that my nerves will subside once I commence the test helps mitigate my nerves, and strangely enough, knowing that it will only last so long and afterwards I will feel relieve can also help.