Triathlons are extremely tough and exhausting on the body. You are putting yourself through metabolic fatigue (energy stores), environmental fatigue (climate), neurological (motor drive/motivation) and psychological fatigue (stress). Due to this, you must be well prepared in your training, the event day itself and your recovery process to get ready for your next.
In order to improve your performance and outcomes, you must overload your body for the adaptations and improvements to occur. The recovery phase in this must be a huge focus as well in order to prevent over-training and fatigue. This includes rest, nutrition and of course hydration. Know how much your body can take and gradually increase the load with rest allowing for these adaptations to occur.
For race day preparation, make sure you start fuelling your body 48 hours before the race. This includes a high carbohydrate diet to store this in your muscles ready for use. The morning of the race, make sure you eat at least 2.5hrs to 3hrs before the race. Know your course inside and out including the transition area. Be prepared with fluids including a water and Gatorade mix to assist with rehydrating fluids and electrolytes.
Post-race, have a high-GI sugary snack within the first 30 minutes. Then a protein, low GI meal 1-2 hours afterwards. If possible, a slow but active recovery can help your muscles recovery. This could include a slow ride on the wind trainer or walking in the pool/ocean. And always allow rest for your body to restore all four fatigued systems and for adaptations to occur.
Good luck for your race day!