What worked for me was relaxing the day before my test day (today for you, OP). I only casually browsed through SDN, looking at other posters' questions to help jog my memory on various subjects. I also got plenty of sleep, I think 8 hours to be exact. I then ate a nice, balanced breakfast the following morning with lots of high energy food (nuts work wonders, and bananas help too). I avoided any type of caffeine because I didn't want to run into any problems with an unexpected bathroom break. That's where natural energy in your breakfast will come into play.
Going to the test center really made me nervous. To help calm me down, I went through some of my notes for the Biology section. I probably didn't retain anything useful, but the act of me doing something useful for the DAT calmed me down. The test itself was very much like practice testing in terms of format. However, the stress and pressure is much more. Right before I started the real test, I used the introductory 15 minute tutorial to just calm down. I did a few breathing exercises and meditated a bit to help focus my mind on the DAT that was about to start (my martial arts background came in plenty of use here). I tuned everything and everyone out, I put on those noise cancelling headphones, and I started my test. Just think positively and reflect on all the hard work that you have done to motivate yourself and give yourself confidence.
Good luck and I'm sure you will do well!