A few things I've learned over the years have helped me be more fully race ready at the startline.
A couple of weeks out, maintain a good amount of stretching and yoga if it's part of your routine. Plenty of sleep will also aid in boosting recovery from workouts. As training load reduces on the lead-in to race day, these 2 things will be of great help in being fresh and flexible for the race.
Dietary: "carb loading" is quite a big subject on its own. I will just say here to avoid overdoing it. A few extra carbs 2-4 days prior should be fine.
Study the course so you know exactly what's coming, and which direction the next corner takes. This way, the runner won't run a metre further than needed. I find that the brain doesn't function much on a conscious level during a race, so it's best to pre-load it with course info and race plan.
Day before: have everything you'll need ready for race day. Pack your bag, put your race clothes in a pile to put on as soon as you're out of bed. Check the hourly weather forecast. Try to dress for the race rather than the startline: in other words, not too warm. While it might be 5 degrees at sunrise, by 10 am it could be 15 degrees and rising.
Race day, allow plenty of time to get to the start. Early is always better, leave 15 minutes earlier than planned. I wear my race shoes on my way there to allow my feet to get truly settled into them and ensure no hurried re-tying just before the starting horn.
Biological matters. Every runner has at some point sung the section from Bohemian Rhapsody "let me go!", including myself during my first marathon. To avoid this, I go: as soon as I wake up, after breakfast, just before I leave home, as soon as I arrive at the venue, and once more before the lines start getting long. This is a lot of visits, however it means starting with greater confidence that there's one less thing to worry about. Furthermore, for marathons, most runners will be finishing mid-morning.
I also try to steer clear of heavy foods the day before.
Race - have an aim and pace in mind, as it's easy to go too fast, especially at the start. Embrace the day and encouragement from the crowd. It might however be better to ignore anyone who says "nearly there!" unless there's less than 1km left.
Take onboard fluids with electrolytes regularly, and start early, especially with the longer distances.
When the going gets tough towards the end, studying the course can come in useful. Memorial Drive may provide difficult moments for many runners. During my first marathon, I broke down that last 10k into sections, telling myself to get to the next bridge - some 1km apart, others a lot closer.
After finishing, try to avoid sitting down as long as you can. All your leg muscles want to do is contract, remaining on your feet will lessen the effect. This has worked extremely well for me over the years - walking is so much harder if you sir for a while soon after the finish line.
A decent lunch can be guilt free - if you've run the marathon or ultra, any and all food is good for you (IMO).
There's nothing like race day, let's go get it.