This is my recommendation before a presentation or anything where you’ll be talking to people: Don’t look in the mirror beforehand.
Unless you’re one of those people that is so in love with themselves that they only see perfection, chances are when you look in the mirror you’ll see at least one imperfection. And chances are that imperfection is probably not fixable in the next 10 minutes. And chances are you will obsess about it and be self conscious of it through the whole presentation, performance, date, etc.
I used to get eczema on my face when I was nervous (all the time), so I took a year off looking in the mirror. I learned to shave by feel, stare downward during elevator rides, and drive recklessly. When I finally did look in the mirror again, a funny thing happened. I smiled. I was happy to see my face. Red blotches or not, I missed that guy. (The red blotches were, of course, lessened by my lack of obsession about them.)
I’m not saying, don’t check and see if you have mustard on your shirt… no, I guess that is what I’m saying. Don’t check to see if you have mustard on your shirt. How is that knowledge going to help you? You gonna try to rinse it out and walk in there with a wet mustard stain? Huh? Is that what you’re gonna do? Pffft. Good luck with that.
Ignorance is bliss. Sure you might have a giant zit on your face, but who cares? If you looked in the mirror and saw it, you would care. Too much. Luckily you’re blissfully ignorant of the mustard stains, zits, messed up hair, ill-fitting clothing, which frankly, make you seem more real and sincere than some clean cut perfect person.
Is this avoiding the bigger issue of the fact that we all obsess about our looks? Yeah probably. But I’m not sure a big presentation is the time to deal with that particular issue. Save that for alone time crying in front of the mirror.