Before you go to the field?
The night before drink a glass water before bed and as soon as rising. The longest period of time a person goes without drinking is when asleep. So, most people wake up dehydrated to some level. Sip on water to get hydrated throughout the morning. Get a good nights sleep. Eat a substantial breakfast. Hitting a drive through 5 minutes before you get to the field is not ideal but better than not eating.
Some people are very into their own rituals, some are not. When you arrive at the field and get your spikes on you should already be starting the process of mentally focusing. Visualize your pitches the night before.....FB on the corner, change up middle in and dropping for a swing and miss, nasty CB or whatever your through. See it first in your mind.