As a triathlete, I push my body and mind to the limit every time I train. But even with all the training that I do, race day is still an intimidating and nerve-wracking experience. That is why I have put considerable thought into what routine I need to go through to arrive at the race as prepared and mentally collected as possible.
My routine includes avoiding going to the race site or being around too many of my competitors too close to the race to avoid getting to nervous. I also try to get into the water or on the road as early as possible. To me. the most important thing is to stay in a calming environment for as long as possible leading up to the race.
Find out what trips you up before your race so that you can personalize your pre-race routine to optimize your strengths. If being around too many people gets you nervous, then postpone any meet-ups or hangouts and get some alone time beforehand. If the opposite is true and surrounding yourself with loved ones gets you pumped up and ready for the race, then make sure you invite a few people to come watch you and spend time with them before you get ready for your big day.