Eat right. A presentation is physical so it is important to be well hydrated and nourished. But don’t eat and drink everything put in front of you.
Leading up to your presentation, try to go high on protein (good energy), low on dairy (harder to digest and can coat your throat). Try not to eat immediately before presenting. Give yourself a good 20 minute interval between food and presenting if you can. But eat well balanced meals the day(s) before. I you do not have the luxury of time, it is more important to have at least some sustenance to carry you through and not forego food and risk feeling faint. So eat a little, eat it slowly. Hydrating is always good at any time.
Avoid drugs (unless prescribed by a Doctor) or alcohol. Some people think alcohol relaxes them but there are better ways to relax. We are professionals. You would not want to learn that your surgeon or airline pilot takes a little nip of whiskey or vodka before your surgery or lift off just to relax and calm their nerves?
Again water is usually best: before, during, and after.